docs: update documentation and implementation plan

Update the API surface and implementation plan to reflect architectural
changes, including new iterator overloads, system design refinements,
and the introduction of a source generator.

Directory.Build.props

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>12</LangVersion>
+  </PropertyGroup>
+</Project>

Game.Blackjack.Tests/Game.Blackjack.Tests.csproj

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <RootNamespace>Game.Blackjack.Tests</RootNamespace>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="FluentAssertions" Version="6.12.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Game.Blackjack\Blackjack.csproj" />
+  </ItemGroup>
+
+</Project>

Game.Blackjack.Tests/GameFlowTests.cs

@@ -0,0 +1,266 @@
+using FluentAssertions;
+using Game.Blackjack;
+using OECS;
+using Xunit;
+
+namespace Game.Blackjack.Tests;
+
+public class GameFlowTests
+{
+    [Fact]
+    public void NewGame_StartsInBettingPhase()
+    {
+        var (world, _) = SetupGame();
+
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.Betting);
+        state.Chips.Should().Be(100);
+        state.RoundNumber.Should().Be(1);
+    }
+
+    [Fact]
+    public void PlaceBet_AdvancesToDealing()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.PlayerTurn); // Dealing → PlayerTurn happens automatically
+        state.CurrentBet.Should().Be(10);
+        state.Chips.Should().Be(90);
+    }
+
+    [Fact]
+    public void PlaceBet_RejectsInsufficientChips()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 200 });
+        group.RunLogical();
+
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.Betting); // Should not advance
+        state.Chips.Should().Be(100);
+    }
+
+    [Fact]
+    public void PlaceBet_RejectsZeroOrNegative()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 0 });
+        group.RunLogical();
+        world.ReadSingleton<GameState>().Phase.Should().Be(GamePhase.Betting);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = -5 });
+        group.RunLogical();
+        world.ReadSingleton<GameState>().Phase.Should().Be(GamePhase.Betting);
+    }
+
+    [Fact]
+    public void Dealing_CreatesDeckAndHands()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        // Should have a deck entity.
+        var deckEntity = FindEntity<Deck>(world);
+        deckEntity.Should().NotBe(Entity.Null);
+
+        // Should have player and dealer hand entities.
+        var playerHand = FindEntity<PlayerHand>(world);
+        playerHand.Should().NotBe(Entity.Null);
+
+        var dealerHand = FindEntity<DealerHand>(world);
+        dealerHand.Should().NotBe(Entity.Null);
+    }
+
+    [Fact]
+    public void Dealing_DealsTwoCardsToEach()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        var playerHand = FindEntity<PlayerHand>(world);
+        var dealerHand = FindEntity<DealerHand>(world);
+
+        CountCardsInHand(world, playerHand).Should().Be(2);
+        CountCardsInHand(world, dealerHand).Should().Be(2);
+    }
+
+    [Fact]
+    public void Hit_DrawsOneCard()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        var playerHand = FindEntity<PlayerHand>(world);
+        var before = CountCardsInHand(world, playerHand);
+
+        world.Commands.Enqueue(new HitCommand());
+        group.RunLogical();
+
+        var after = CountCardsInHand(world, playerHand);
+        after.Should().Be(before + 1);
+    }
+
+    [Fact]
+    public void Stand_AdvancesToDealerTurn()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        world.Commands.Enqueue(new StandCommand());
+        group.RunLogical();
+
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.RoundOver);
+    }
+
+    [Fact]
+    public void NewRound_ResetsPhase()
+    {
+        var (world, group) = SetupGame();
+
+        // Play a round.
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+        world.Commands.Enqueue(new StandCommand());
+        group.RunLogical();
+
+        // Start new round.
+        world.Commands.Enqueue(new NewRoundCommand());
+        group.RunLogical();
+
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.Betting);
+        state.Result.Should().Be(RoundResult.None);
+    }
+
+    [Fact]
+    public void DeterministicSeed_ProducesSameDeal()
+    {
+        var (world1, group1) = SetupGame(seed: 42);
+        world1.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group1.RunLogical();
+        var cards1 = GetAllCards(world1);
+
+        var (world2, group2) = SetupGame(seed: 42);
+        world2.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group2.RunLogical();
+        var cards2 = GetAllCards(world2);
+
+        cards1.Should().Equal(cards2);
+    }
+
+    [Fact]
+    public void PlayerBust_LosesBet()
+    {
+        // Use a seed that produces a bust-prone hand, then hit repeatedly.
+        var (world, group) = SetupGame(seed: 12345);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        // Hit until bust or stand.
+        while (world.ReadSingleton<GameState>().Phase == GamePhase.PlayerTurn)
+        {
+            world.Commands.Enqueue(new HitCommand());
+            group.RunLogical();
+        }
+
+        // Game should have resolved.
+        var state = world.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.RoundOver);
+    }
+
+    [Fact]
+    public void Serialization_RoundTrips()
+    {
+        var (world, group) = SetupGame(seed: 42);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        using var stream = new MemoryStream();
+        WorldSerializer.Save(world, stream);
+        stream.Position = 0;
+
+        var world2 = new World();
+        WorldSerializer.Load(world2, stream);
+
+        var state = world2.ReadSingleton<GameState>();
+        state.Chips.Should().Be(90);
+        state.CurrentBet.Should().Be(10);
+        state.RoundNumber.Should().Be(1);
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame(uint seed = 42)
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new DeckSetupSystem());
+        group.Add(new DealSystem());
+        group.Add(new PlayerBustCheckSystem());
+        group.Add(new DealerSystem());
+
+        world.SetSingleton(new GameState
+        {
+            Phase = GamePhase.Betting,
+            Result = RoundResult.None,
+            Chips = 100,
+            CurrentBet = 0,
+            RoundNumber = 1,
+            Seed = seed
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    private static Entity FindEntity<T>(World world) where T : struct
+    {
+        using var iter = world.Select<T>();
+        while (iter.MoveNext())
+        {
+            if (iter.CurrentEntity != World.SingletonEntity)
+                return iter.CurrentEntity;
+        }
+        return Entity.Null;
+    }
+
+    private static int CountCardsInHand(World world, Entity handEntity)
+    {
+        // Holds relationship: Source = card entity, Target = hand entity.
+        // GetSources returns all card entities that have a Holds pointing to handEntity.
+        return world.GetSources<Holds>(handEntity).Count;
+    }
+
+    private static List<(Suit, Rank)> GetAllCards(World world)
+    {
+        var cards = new List<(Suit, Rank)>();
+        var query = world.Query().With<Card>().Build();
+        world.ForEach(query, (Entity e, ref Card card) =>
+        {
+            cards.Add((card.Suit, card.Rank));
+        });
+        cards.Sort((a, b) =>
+        {
+            int cmp = a.Item1.CompareTo(b.Item1);
+            return cmp != 0 ? cmp : a.Item2.CompareTo(b.Item2);
+        });
+        return cards;
+    }
+}

Game.Blackjack.Tests/PlayTests.cs

@@ -0,0 +1,381 @@
+using FluentAssertions;
+using Game.Blackjack;
+using OECS;
+using R3;
+using Xunit;
+
+namespace Game.Blackjack.Tests;
+
+public class PlayTests
+{
+    private const int StartingChips = 100;
+    private const int BetAmount = 10;
+
+    [Fact]
+    public void Play_BasicStrategy()
+    {
+        var log = new PlayLog();
+        var (world, group) = SetupGame(seed: 42);
+
+        world.ObserveComponentChanges<Card>().Subscribe(change =>
+        {
+            var card = world.ReadComponent<Card>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Card = {card} on {change.Entity}");
+        });
+        world.ObserveComponentChanges<GameState>().Subscribe(change =>
+        {
+            var state = world.ReadComponent<GameState>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} GameState = {state}");
+        });
+
+        var agent = new BasicStrategyAgent();
+        log.Header = $"Blackjack: Basic Strategy (seed=42, agent={agent})";
+
+        RunUntilDone(world, group, agent, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("blackjack_basic_strategy.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    [Fact]
+    public void Play_RandomAgent()
+    {
+        var log = new PlayLog();
+        var (world, group) = SetupGame(seed: 123);
+
+        world.ObserveComponentChanges<Card>().Subscribe(change =>
+        {
+            var card = world.ReadComponent<Card>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Card = {card} on {change.Entity}");
+        });
+        world.ObserveComponentChanges<GameState>().Subscribe(change =>
+        {
+            var state = world.ReadComponent<GameState>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} GameState = {state}");
+        });
+
+        var agent = new RandomBlackjackAgent();
+        log.Header = $"Blackjack: Random Agent (seed=123, agent={agent})";
+
+        RunUntilDone(world, group, agent, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("blackjack_random_agent.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    [Fact]
+    public void Play_WeightedPool()
+    {
+        var log = new PlayLog();
+        var (world, group) = SetupGame(seed: 77);
+
+        world.ObserveComponentChanges<Card>().Subscribe(change =>
+        {
+            var card = world.ReadComponent<Card>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Card = {card} on {change.Entity}");
+        });
+        world.ObserveComponentChanges<GameState>().Subscribe(change =>
+        {
+            var state = world.ReadComponent<GameState>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} GameState = {state}");
+        });
+
+        var pool = new WeightedAgentPool();
+        pool.Add(new BasicStrategyAgent(), 7);
+        pool.Add(new RandomBlackjackAgent(), 3);
+
+        var agent = pool.Pick();
+        log.Header = $"Blackjack: Weighted Pool (seed=77, agent={agent})";
+
+        RunUntilDone(world, group, agent, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("blackjack_weighted_pool.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    // ── Play Log ──────────────────────────────────────────────────────
+
+    private sealed class PlayLog
+    {
+        public string Header { get; set; } = "";
+        public readonly List<string> RoundSummaries = new();
+        public readonly List<string> Decisions = new();
+        public readonly List<string> Reactivity = new();
+        public string FinalSnapshot { get; set; } = "";
+
+        public string Build()
+        {
+            var sb = new System.Text.StringBuilder();
+            sb.AppendLine(Header);
+            sb.AppendLine(new string('=', Header.Length));
+            sb.AppendLine();
+            sb.AppendLine("--- Round Summaries ---");
+            for (int i = 0; i < RoundSummaries.Count; i++)
+                sb.AppendLine($"  Round {i + 1}: {RoundSummaries[i]}");
+            sb.AppendLine();
+            sb.AppendLine("--- Decisions ---");
+            for (int i = 0; i < Decisions.Count; i++)
+                sb.AppendLine($"  {i + 1}. {Decisions[i]}");
+            sb.AppendLine();
+            sb.AppendLine("--- Reactivity ---");
+            foreach (var entry in Reactivity)
+                sb.AppendLine($"  {entry}");
+            sb.AppendLine();
+            sb.AppendLine("--- Final State ---");
+            sb.AppendLine(FinalSnapshot);
+            return sb.ToString();
+        }
+    }
+
+    // ── Round Runner ──────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame(uint seed)
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new DeckSetupSystem());
+        group.Add(new DealSystem());
+        group.Add(new PlayerBustCheckSystem());
+        group.Add(new DealerSystem());
+
+        world.SetSingleton(new GameState
+        {
+            Phase = GamePhase.Betting,
+            Result = RoundResult.None,
+            Chips = StartingChips,
+            CurrentBet = 0,
+            RoundNumber = 1,
+            Seed = seed
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    /// <summary>
+    /// Runs rounds until the player runs out of chips (can't afford minimum bet)
+    /// or doubles their starting chips.
+    /// </summary>
+    private static void RunUntilDone(World world, SystemGroup group, IBlackjackAgent agent, PlayLog log)
+    {
+        int totalRounds = 0;
+        int wins = 0;
+        int losses = 0;
+        int pushes = 0;
+
+        while (true)
+        {
+            var state = world.ReadSingleton<GameState>();
+            int chips = state.Chips;
+
+            // Stop conditions.
+            if (chips < BetAmount)
+            {
+                log.Header += $" — BUSTED after {totalRounds} rounds";
+                break;
+            }
+            if (chips >= StartingChips * 2)
+            {
+                log.Header += $" — DOUBLED after {totalRounds} rounds";
+                break;
+            }
+
+            // Safety: max 200 rounds to prevent infinite loops.
+            if (totalRounds >= 200)
+            {
+                log.Header += $" — MAX ROUNDS ({totalRounds})";
+                break;
+            }
+
+            // Place bet and run dealing.
+            world.Commands.Enqueue(new PlaceBetCommand { Amount = BetAmount });
+            group.RunLogical();
+
+            // Player turn: agent decides hit/stand.
+            int roundHits = 0;
+            while (world.ReadSingleton<GameState>().Phase == GamePhase.PlayerTurn)
+            {
+                var total = HandUtil.CalculateHand(world, new PlayerHand());
+                var decision = agent.Decide(world);
+                log.Decisions.Add($"R{totalRounds + 1} Hand={total}, Decision={decision}");
+
+                if (decision == BlackjackDecision.Hit)
+                {
+                    world.Commands.Enqueue(new HitCommand());
+                    roundHits++;
+                }
+                else
+                {
+                    world.Commands.Enqueue(new StandCommand());
+                }
+
+                group.RunLogical();
+            }
+
+            // Record round result.
+            state = world.ReadSingleton<GameState>();
+            int newChips = state.Chips;
+            int delta = newChips - chips;
+            string summary = $"{state.Result} | Bet={BetAmount} | Chips: {chips}→{newChips} ({delta:+0;-#}) | Hits: {roundHits}";
+            log.RoundSummaries.Add(summary);
+
+            switch (state.Result)
+            {
+                case RoundResult.PlayerWin:
+                case RoundResult.DealerBust:
+                    wins++;
+                    break;
+                case RoundResult.DealerWin:
+                case RoundResult.PlayerBust:
+                    losses++;
+                    break;
+                case RoundResult.Push:
+                    pushes++;
+                    break;
+            }
+
+            totalRounds++;
+
+            // Start next round.
+            world.Commands.Enqueue(new NewRoundCommand());
+            group.RunLogical();
+        }
+
+        log.Header += $" | W:{wins} L:{losses} P:{pushes}";
+    }
+
+    // ── Snapshot ──────────────────────────────────────────────────────
+
+    private static string SnapshotWorld(World world)
+    {
+        var sb = new System.Text.StringBuilder();
+        var state = world.ReadSingleton<GameState>();
+        sb.AppendLine($"Phase: {state.Phase}");
+        sb.AppendLine($"Chips: {state.Chips}, Bet: {state.CurrentBet}");
+        sb.AppendLine($"Round: {state.RoundNumber}");
+        sb.AppendLine($"Result: {state.Result}");
+
+        var playerHand = FindEntity<PlayerHand>(world);
+        if (playerHand != Entity.Null)
+        {
+            sb.AppendLine($"Player ({HandUtil.CalculateHand(world, new PlayerHand())}):");
+            foreach (var card in GetHandCards(world, playerHand))
+                sb.AppendLine($"  {card}");
+        }
+
+        var dealerHand = FindEntity<DealerHand>(world);
+        if (dealerHand != Entity.Null)
+        {
+            sb.AppendLine($"Dealer ({HandUtil.CalculateHand(world, new DealerHand())}):");
+            foreach (var card in GetHandCards(world, dealerHand))
+                sb.AppendLine($"  {card}");
+        }
+
+        return sb.ToString().TrimEnd();
+    }
+
+    private static Entity FindEntity<T>(World world) where T : struct
+    {
+        using var iter = world.Select<T>();
+        while (iter.MoveNext())
+            if (iter.CurrentEntity != World.SingletonEntity)
+                return iter.CurrentEntity;
+        return Entity.Null;
+    }
+
+    private static List<string> GetHandCards(World world, Entity hand)
+    {
+        var cards = new List<string>();
+        foreach (var cardEntity in world.GetSources<Holds>(hand))
+        {
+            var card = world.ReadComponent<Card>(cardEntity);
+            cards.Add($"{card.Rank} of {card.Suit}");
+        }
+        return cards;
+    }
+
+    // ── File I/O ──────────────────────────────────────────────────────
+
+    private static string SavePlayLog(string filename, PlayLog log)
+    {
+        var dir = Path.Combine(AppContext.BaseDirectory, "playlogs");
+        Directory.CreateDirectory(dir);
+        var path = Path.Combine(dir, filename);
+        File.WriteAllText(path, log.Build());
+        return path;
+    }
+
+    private static void ReadAndVerify(string path)
+    {
+        var content = File.ReadAllText(path);
+
+        content.Should().Contain("--- Round Summaries ---");
+        content.Should().Contain("--- Decisions ---");
+        content.Should().Contain("--- Reactivity ---");
+        content.Should().Contain("--- Final State ---");
+        content.Should().Contain("Result:");
+    }
+
+    // ── Agents ────────────────────────────────────────────────────────
+
+    private enum BlackjackDecision { Hit, Stand }
+
+    private interface IBlackjackAgent
+    {
+        BlackjackDecision Decide(World world);
+    }
+
+    private sealed class BasicStrategyAgent : IBlackjackAgent
+    {
+        public BlackjackDecision Decide(World world)
+        {
+            var total = HandUtil.CalculateHand(world, new PlayerHand());
+            return total < 17 ? BlackjackDecision.Hit : BlackjackDecision.Stand;
+        }
+        public override string ToString() => "BasicStrategy";
+    }
+
+    private sealed class RandomBlackjackAgent : IBlackjackAgent
+    {
+        private static readonly Random _rng = new();
+        public BlackjackDecision Decide(World world)
+        {
+            var total = HandUtil.CalculateHand(world, new PlayerHand());
+            if (total >= 21) return BlackjackDecision.Stand;
+            return _rng.Next(2) == 0 ? BlackjackDecision.Hit : BlackjackDecision.Stand;
+        }
+        public override string ToString() => "Random";
+    }
+
+    private sealed class WeightedAgentPool
+    {
+        private readonly List<(IBlackjackAgent Agent, int Weight)> _agents = new();
+        private int _totalWeight;
+        private static readonly Random _rng = new();
+
+        public void Add(IBlackjackAgent agent, int weight)
+        {
+            _agents.Add((agent, weight));
+            _totalWeight += weight;
+        }
+
+        public IBlackjackAgent Pick()
+        {
+            int roll = _rng.Next(_totalWeight);
+            int cumulative = 0;
+            foreach (var (agent, weight) in _agents)
+            {
+                cumulative += weight;
+                if (roll < cumulative)
+                    return agent;
+            }
+            return _agents[^1].Agent;
+        }
+    }
+}

Game.Blackjack.Tests/SnapshotTests.cs

@@ -0,0 +1,259 @@
+using FluentAssertions;
+using Game.Blackjack;
+using OECS;
+using R3;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Game.Blackjack.Tests;
+
+/// <summary>
+/// Baseline snapshot and log tests for Blackjack.
+/// Captures the world state as text and R3 change logs so they can be
+/// reviewed manually and compared across changes.
+/// </summary>
+public class SnapshotTests
+{
+    private readonly ITestOutputHelper _output;
+
+    public SnapshotTests(ITestOutputHelper output)
+    {
+        _output = output;
+    }
+
+    [Fact]
+    public void Snapshot_InitialState()
+    {
+        var (world, _) = SetupGame();
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("Phase: Betting");
+        snapshot.Should().Contain("Chips: 100");
+        snapshot.Should().Contain("Round: 1");
+    }
+
+    [Fact]
+    public void Snapshot_AfterDeal()
+    {
+        var (world, group) = SetupGame(seed: 42);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("Phase: PlayerTurn");
+        snapshot.Should().Contain("Chips: 90");
+        snapshot.Should().Contain("Bet: 10");
+        snapshot.Should().Contain("Player hand:");
+        snapshot.Should().Contain("Dealer hand:");
+        snapshot.Should().Contain("Cards in deck:");
+    }
+
+    [Fact]
+    public void Snapshot_AfterHit()
+    {
+        var (world, group) = SetupGame(seed: 42);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        world.Commands.Enqueue(new HitCommand());
+        group.RunLogical();
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        // With seed 42, one hit may bust. Just verify the game resolved.
+        snapshot.Should().NotContain("Phase: Dealing");
+    }
+
+    [Fact]
+    public void Snapshot_AfterStand()
+    {
+        var (world, group) = SetupGame(seed: 42);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        world.Commands.Enqueue(new StandCommand());
+        group.RunLogical();
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("Phase: RoundOver");
+        snapshot.Should().Contain("Result:");
+    }
+
+    [Fact]
+    public void Log_ReactivityDuringRound()
+    {
+        var (world, group) = SetupGame(seed: 42);
+        var log = new List<string>();
+
+        world.ObserveEntityChanges().Subscribe(change =>
+        {
+            log.Add($"[entity] {change}");
+        });
+
+        world.ObserveComponentChanges<Card>().Subscribe(change =>
+        {
+            log.Add($"[card] {change}");
+        });
+
+        world.ObserveComponentChanges<GameState>().Subscribe(change =>
+        {
+            log.Add($"[gamestate] {change}");
+        });
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        world.Commands.Enqueue(new StandCommand());
+        group.RunLogical();
+
+        var logText = string.Join("\n", log);
+        _output.WriteLine(logText);
+
+        // Should have entity creations (deck, hands, cards) and component changes.
+        log.Should().Contain(l => l.Contains("EntityAdded"));
+        log.Should().Contain(l => l.Contains("ComponentAdded") && l.Contains("Card"));
+        log.Should().Contain(l => l.Contains("ComponentModified") && l.Contains("GameState"));
+    }
+
+    [Fact]
+    public void Serialization_RoundTrip_PreservesCoreState()
+    {
+        var (world, group) = SetupGame(seed: 42);
+
+        world.Commands.Enqueue(new PlaceBetCommand { Amount = 10 });
+        group.RunLogical();
+
+        var before = SnapshotWorld(world);
+
+        using var stream = new MemoryStream();
+        WorldSerializer.Save(world, stream);
+        stream.Position = 0;
+
+        var world2 = new World();
+        WorldSerializer.Load(world2, stream);
+
+        var after = SnapshotWorld(world2);
+
+        _output.WriteLine("=== Before ===");
+        _output.WriteLine(before);
+        _output.WriteLine("=== After ===");
+        _output.WriteLine(after);
+
+        // Core game state must round-trip.
+        var state = world2.ReadSingleton<GameState>();
+        state.Phase.Should().Be(GamePhase.PlayerTurn);
+        state.Chips.Should().Be(90);
+        state.CurrentBet.Should().Be(10);
+        state.RoundNumber.Should().Be(1);
+
+        // Card entities must be preserved.
+        after.Should().Contain("Card entities: 52");
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame(uint seed = 42)
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new DeckSetupSystem());
+        group.Add(new DealSystem());
+        group.Add(new PlayerBustCheckSystem());
+        group.Add(new DealerSystem());
+
+        world.SetSingleton(new GameState
+        {
+            Phase = GamePhase.Betting,
+            Result = RoundResult.None,
+            Chips = 100,
+            CurrentBet = 0,
+            RoundNumber = 1,
+            Seed = seed
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    /// <summary>
+    /// Produces a human-readable textual snapshot of the world state.
+    /// </summary>
+    private static string SnapshotWorld(World world)
+    {
+        var sb = new System.Text.StringBuilder();
+
+        // GameState singleton.
+        var state = world.ReadSingleton<GameState>();
+        sb.AppendLine($"Phase: {state.Phase}");
+        sb.AppendLine($"Chips: {state.Chips}");
+        sb.AppendLine($"Bet: {state.CurrentBet}");
+        sb.AppendLine($"Round: {state.RoundNumber}");
+        sb.AppendLine($"Result: {state.Result}");
+        sb.AppendLine($"Seed: {state.Seed}");
+
+        // Deck entity.
+        var deckEntity = FindEntity<Deck>(world);
+        if (deckEntity != Entity.Null)
+        {
+            var cardsInDeck = world.GetSources<InDeck>(deckEntity).ToList();
+            sb.AppendLine($"Cards in deck: {cardsInDeck.Count}");
+        }
+
+        // Player hand.
+        var playerHand = FindEntity<PlayerHand>(world);
+        if (playerHand != Entity.Null)
+        {
+            var cards = world.GetSources<Holds>(playerHand).ToList();
+            sb.AppendLine($"Player hand: {cards.Count} cards");
+            foreach (var cardEntity in cards)
+            {
+                var card = world.ReadComponent<Card>(cardEntity);
+                sb.AppendLine($"  {card.Rank} of {card.Suit}");
+            }
+        }
+
+        // Dealer hand.
+        var dealerHand = FindEntity<DealerHand>(world);
+        if (dealerHand != Entity.Null)
+        {
+            var cards = world.GetSources<Holds>(dealerHand).ToList();
+            sb.AppendLine($"Dealer hand: {cards.Count} cards");
+            foreach (var cardEntity in cards)
+            {
+                var card = world.ReadComponent<Card>(cardEntity);
+                sb.AppendLine($"  {card.Rank} of {card.Suit}");
+            }
+        }
+
+        // All entities.
+        int entityCount = 0;
+        using (var iter = world.Select<Card>())
+        {
+            while (iter.MoveNext()) entityCount++;
+        }
+        sb.AppendLine($"Card entities: {entityCount}");
+
+        return sb.ToString().TrimEnd();
+    }
+
+    private static Entity FindEntity<T>(World world) where T : struct
+    {
+        using var iter = world.Select<T>();
+        while (iter.MoveNext())
+        {
+            if (iter.CurrentEntity != World.SingletonEntity)
+                return iter.CurrentEntity;
+        }
+        return Entity.Null;
+    }
+}

Game.Blackjack/Blackjack.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <RootNamespace>Game.Blackjack</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\OECS\OECS.csproj" />
+  </ItemGroup>
+
+  <!-- Source generator: referenced as a built DLL. -->
+  <Target Name="EnsureSourceGenBuilt" BeforeTargets="CoreCompile">
+    <MSBuild Projects="..\OECS.SourceGen\OECS.SourceGen.csproj"
+             Targets="Build"
+             Properties="Configuration=$(Configuration)" />
+  </Target>
+
+  <ItemGroup>
+    <Analyzer Include="..\OECS.SourceGen\bin\$(Configuration)\netstandard2.0\OECS.SourceGen.dll" />
+  </ItemGroup>
+
+</Project>

Game.Blackjack/Commands/HitCommand.cs

@@ -1,7 +1,7 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Player hits: draws one card from the deck to the player's hand.

Game.Blackjack/Commands/NewRoundCommand.cs

@@ -1,7 +1,7 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Starts a new round after the previous one ended.
@@ -46,6 +46,7 @@ public struct NewRoundCommand : ICommand
             }
         }
 
+        state.RoundNumber++;
         state.Phase = GamePhase.Betting;
         state.Result = RoundResult.None;
         world.MarkModified<GameState>(World.SingletonEntity);

Game.Blackjack/Commands/PlaceBetCommand.cs

@@ -1,7 +1,7 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Places a bet and advances the game to the dealing phase.

Game.Blackjack/Commands/StandCommand.cs

@@ -1,7 +1,7 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Player stands: advance to the dealer's turn.

Game.Blackjack/Components/Card.cs

@@ -1,13 +1,13 @@
 using MessagePack;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// A playing card with a suit and rank.
 /// One entity per card.
 /// </summary>
 [MessagePackObject]
-public struct Card
+public record struct Card
 {
     [Key(0)] public Suit Suit;
     [Key(1)] public Rank Rank;

Game.Blackjack/Components/DealerHand.cs

@@ -1,6 +1,6 @@
 using MessagePack;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Tag component marking the dealer's hand entity.

Game.Blackjack/Components/Deck.cs

@@ -1,6 +1,6 @@
 using MessagePack;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Tag component marking the deck entity.

Game.Blackjack/Components/Holds.cs

@@ -1,14 +1,14 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Relationship from a hand entity to a card entity,
 /// representing that the hand holds this card.
 /// </summary>
 [MessagePackObject]
-public struct Holds : IRelationship
+public record struct Holds : IRelationship
 {
     [Key(0)] public Entity Source { get; set; }
     [Key(1)] public Entity Target { get; set; }

Game.Blackjack/Components/InDeck.cs

@@ -1,14 +1,14 @@
 using MessagePack;
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Relationship from a card entity to the deck entity,
 /// representing that the card is in the deck.
 /// </summary>
 [MessagePackObject]
-public struct InDeck : IRelationship
+public record struct InDeck : IRelationship
 {
     [Key(0)] public Entity Source { get; set; }
     [Key(1)] public Entity Target { get; set; }

Game.Blackjack/Components/PlayerHand.cs

@@ -1,6 +1,6 @@
 using MessagePack;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Tag component marking the player's hand entity.

Game.Blackjack/Components/Rank.cs

@@ -1,4 +1,4 @@
-namespace Blackjack;
+namespace Game.Blackjack;
 
 public enum Rank : byte
 {

Game.Blackjack/Components/Suit.cs

@@ -1,4 +1,4 @@
-namespace Blackjack;
+namespace Game.Blackjack;
 
 public enum Suit : byte
 {

Game.Blackjack/Mulberry32.cs

@@ -1,4 +1,4 @@
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Mulberry32 PRNG — a fast, high-quality 32-bit random number generator.

Game.Blackjack/Singletons/GamePhase.cs

@@ -1,4 +1,4 @@
-namespace Blackjack;
+namespace Game.Blackjack;
 
 public enum GamePhase : byte
 {

Game.Blackjack/Singletons/GameState.cs

@@ -1,12 +1,12 @@
 using MessagePack;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Global game state stored on the singleton entity.
 /// </summary>
 [MessagePackObject]
-public struct GameState
+public record struct GameState
 {
     [Key(0)] public GamePhase Phase;
     [Key(1)] public RoundResult Result;

Game.Blackjack/Singletons/RoundResult.cs

@@ -1,4 +1,4 @@
-namespace Blackjack;
+namespace Game.Blackjack;
 
 public enum RoundResult : byte
 {

Game.Blackjack/Systems/DealSystem.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Deals initial two cards to player and dealer when entering the dealing phase.

Game.Blackjack/Systems/DealerSystem.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Dealer draws cards until reaching 17 or higher,

Game.Blackjack/Systems/DeckSetupSystem.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Creates the deck (52 cards + deck/hand entities) and shuffles

Game.Blackjack/Systems/HandUtil.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Helper to calculate the blackjack value of a hand.

Game.Blackjack/Systems/PlayerBustCheckSystem.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace Blackjack;
+namespace Game.Blackjack;
 
 /// <summary>
 /// Evaluates the player's hand total after each hit.

Game.TicTacToe.Tests/Game.TicTacToe.Tests.csproj

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <RootNamespace>Game.TicTacToe.Tests</RootNamespace>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="FluentAssertions" Version="6.12.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Game.TicTacToe\TicTacToe.csproj" />
+  </ItemGroup>
+
+</Project>

Game.TicTacToe.Tests/GameFlowTests.cs

@@ -0,0 +1,213 @@
+using FluentAssertions;
+using Game.TicTacToe;
+using OECS;
+using Xunit;
+
+namespace Game.TicTacToe.Tests;
+
+public class GameFlowTests
+{
+    [Fact]
+    public void NewGame_HasEmptyBoard()
+    {
+        var (world, _) = SetupGame();
+
+        var query = world.Query().With<Cell>().Without<Mark>().Build();
+        var emptyCount = 0;
+        world.ForEach(query, (Entity e, ref Cell cell) => emptyCount++);
+
+        emptyCount.Should().Be(9);
+    }
+
+    [Fact]
+    public void PlaceMark_ClaimsCell()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 0, Col = 0 });
+        group.RunLogical();
+
+        var query = world.Query().With<Cell>().With<Mark>().Build();
+        var markedCount = 0;
+        world.ForEach(query, (Entity e, ref Cell cell, ref Mark mark) =>
+        {
+            markedCount++;
+            cell.Row.Should().Be(0);
+            cell.Col.Should().Be(0);
+            mark.Player.Should().Be(Player.X);
+        });
+
+        markedCount.Should().Be(1);
+    }
+
+    [Fact]
+    public void PlaceMark_TogglesPlayer()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 0, Col = 0 }); // X
+        group.RunLogical();
+        world.ReadSingleton<GameState>().CurrentPlayer.Should().Be(Player.O);
+
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 1, Col = 0 }); // O
+        group.RunLogical();
+        world.ReadSingleton<GameState>().CurrentPlayer.Should().Be(Player.X);
+    }
+
+    [Fact]
+    public void PlaceMark_CannotOverwriteClaimedCell()
+    {
+        var (world, group) = SetupGame();
+
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 0, Col = 0 }); // X
+        group.RunLogical();
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 0, Col = 0 }); // O tries same cell
+        group.RunLogical();
+
+        // Only one mark should exist at (0,0), and it should still be X.
+        var query = world.Query().With<Cell>().With<Mark>().Build();
+        var marks = new List<(int Row, int Col, Player Player)>();
+        world.ForEach(query, (Entity e, ref Cell cell, ref Mark mark) =>
+        {
+            marks.Add((cell.Row, cell.Col, mark.Player));
+        });
+
+        marks.Should().ContainSingle()
+            .Which.Should().Be((0, 0, Player.X));
+    }
+
+    [Fact]
+    public void XWins_Row()
+    {
+        var (world, group) = SetupGame();
+
+        // X: (0,0), O: (1,0), X: (0,1), O: (1,1), X: (0,2) → X wins row 0
+        PlayMoves(world, group, (0, 0), (1, 0), (0, 1), (1, 1), (0, 2));
+
+        world.ReadSingleton<GameState>().Status.Should().Be(GameStatus.XWon);
+    }
+
+    [Fact]
+    public void OWins_Column()
+    {
+        var (world, group) = SetupGame();
+
+        // X: (0,0), O: (1,0), X: (0,1), O: (1,1), X: (2,2), O: (1,2) → O wins col 0
+        PlayMoves(world, group, (0, 0), (1, 0), (0, 1), (1, 1), (2, 2), (1, 2));
+
+        world.ReadSingleton<GameState>().Status.Should().Be(GameStatus.OWon);
+    }
+
+    [Fact]
+    public void XWins_Diagonal()
+    {
+        var (world, group) = SetupGame();
+
+        // X: (0,0), O: (1,0), X: (1,1), O: (1,2), X: (2,2) → X wins diagonal
+        PlayMoves(world, group, (0, 0), (1, 0), (1, 1), (1, 2), (2, 2));
+
+        world.ReadSingleton<GameState>().Status.Should().Be(GameStatus.XWon);
+    }
+
+    [Fact]
+    public void Draw_AllCellsFilled()
+    {
+        var (world, group) = SetupGame();
+
+        // Fill all 9 cells without a winner.
+        // X: (0,0) (0,2) (1,0) (2,1) (2,2)
+        // O: (0,1) (1,1) (1,2) (2,0)
+        PlayMoves(world, group,
+            (0, 0), (0, 1), (0, 2),
+            (1, 1), (1, 0), (1, 2),
+            (2, 1), (2, 0), (2, 2));
+
+        world.ReadSingleton<GameState>().Status.Should().Be(GameStatus.Draw);
+    }
+
+    [Fact]
+    public void MovesAfterGameOver_AreIgnored()
+    {
+        var (world, group) = SetupGame();
+
+        // X wins.
+        PlayMoves(world, group, (0, 0), (1, 0), (0, 1), (1, 1), (0, 2));
+
+        world.ReadSingleton<GameState>().Status.Should().Be(GameStatus.XWon);
+
+        // Try to place another mark — should be ignored.
+        var moveCountBefore = world.ReadSingleton<GameState>().MoveCount;
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 2, Col = 2 });
+        group.RunLogical();
+
+        world.ReadSingleton<GameState>().MoveCount.Should().Be(moveCountBefore);
+    }
+
+    [Fact]
+    public void SaveLoad_RoundTrips()
+    {
+        var (world, group) = SetupGame();
+
+        // Play a few moves.
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 0, Col = 0 });
+        group.RunLogical();
+        world.Commands.Enqueue(new PlaceMarkCommand { Row = 1, Col = 0 });
+        group.RunLogical();
+
+        // Save.
+        using var stream = new MemoryStream();
+        WorldSerializer.Save(world, stream);
+        stream.Position = 0;
+
+        // Load into a new world.
+        var world2 = new World();
+        WorldSerializer.Load(world2, stream);
+
+        // Verify state.
+        var state = world2.ReadSingleton<GameState>();
+        state.CurrentPlayer.Should().Be(Player.X);
+        state.MoveCount.Should().Be(2);
+
+        var query = world2.Query().With<Cell>().With<Mark>().Build();
+        var marks = new List<(int Row, int Col, Player Player)>();
+        world2.ForEach(query, (Entity e, ref Cell cell, ref Mark mark) =>
+        {
+            marks.Add((cell.Row, cell.Col, mark.Player));
+        });
+
+        marks.Should().BeEquivalentTo([(0, 0, Player.X), (1, 0, Player.O)]);
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame()
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new WinCheckSystem());
+
+        // Create the 9 cells.
+        for (int r = 0; r < 3; r++)
+            for (int c = 0; c < 3; c++)
+                world.AddComponent(world.CreateEntity(), new Cell { Row = r, Col = c });
+
+        world.SetSingleton(new GameState
+        {
+            CurrentPlayer = Player.X,
+            Status = GameStatus.Playing,
+            MoveCount = 0
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    private static void PlayMoves(World world, SystemGroup group, params (int Row, int Col)[] moves)
+    {
+        foreach (var (row, col) in moves)
+        {
+            world.Commands.Enqueue(new PlaceMarkCommand { Row = row, Col = col });
+            group.RunLogical();
+        }
+    }
+}

Game.TicTacToe.Tests/PlayTests.cs

@@ -0,0 +1,284 @@
+using FluentAssertions;
+using Game.TicTacToe;
+using OECS;
+using R3;
+using Xunit;
+
+namespace Game.TicTacToe.Tests;
+
+public class PlayTests
+{
+    [Fact]
+    public void Play_GreedyVsRandom()
+    {
+        var log = new PlayLog();
+
+        var (world, group) = SetupGame();
+        var agentX = new GreedyAgent(GreedyAgent.Strategy.WinOrBlock);
+        var agentO = new RandomAgent();
+
+        // Subscribe reactivity.
+        world.ObserveComponentChanges<Mark>().Subscribe(change =>
+        {
+            var mark = world.ReadComponent<Mark>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Mark = {mark} on {change.Entity}");
+        });
+
+        log.Header = "TicTacToe: Greedy X vs Random O";
+
+        RunGame(world, group, agentX, agentO, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("tic_tac_toe_greedy_vs_random.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    [Fact]
+    public void Play_RandomVsRandom()
+    {
+        var log = new PlayLog();
+
+        var (world, group) = SetupGame();
+        var agentX = new RandomAgent();
+        var agentO = new RandomAgent();
+
+        world.ObserveComponentChanges<Mark>().Subscribe(change =>
+        {
+            var mark = world.ReadComponent<Mark>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Mark = {mark} on {change.Entity}");
+        });
+
+        log.Header = "TicTacToe: Random X vs Random O";
+
+        RunGame(world, group, agentX, agentO, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("tic_tac_toe_random_vs_random.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    [Fact]
+    public void Play_GreedyVsGreedy()
+    {
+        var log = new PlayLog();
+
+        var (world, group) = SetupGame();
+        var agentX = new GreedyAgent(GreedyAgent.Strategy.WinOrBlock);
+        var agentO = new GreedyAgent(GreedyAgent.Strategy.WinOrBlock);
+
+        world.ObserveComponentChanges<Mark>().Subscribe(change =>
+        {
+            var mark = world.ReadComponent<Mark>(change.Entity);
+            log.Reactivity.Add($"{change.Kind} Mark = {mark} on {change.Entity}");
+        });
+
+        log.Header = "TicTacToe: Greedy X vs Greedy O";
+
+        RunGame(world, group, agentX, agentO, log);
+
+        log.FinalSnapshot = SnapshotWorld(world);
+
+        var path = SavePlayLog("tic_tac_toe_greedy_vs_greedy.playlog", log);
+        ReadAndVerify(path);
+    }
+
+    // ── Play Log ──────────────────────────────────────────────────────
+
+    private sealed class PlayLog
+    {
+        public string Header { get; set; } = "";
+        public readonly List<string> Moves = new();
+        public readonly List<string> Reactivity = new();
+        public string FinalSnapshot { get; set; } = "";
+
+        public string Build()
+        {
+            var sb = new System.Text.StringBuilder();
+            sb.AppendLine(Header);
+            sb.AppendLine(new string('=', Header.Length));
+            sb.AppendLine();
+            sb.AppendLine("--- Moves ---");
+            for (int i = 0; i < Moves.Count; i++)
+                sb.AppendLine($"  {i + 1}. {Moves[i]}");
+            sb.AppendLine();
+            sb.AppendLine("--- Reactivity ---");
+            foreach (var entry in Reactivity)
+                sb.AppendLine($"  {entry}");
+            sb.AppendLine();
+            sb.AppendLine("--- Final Board ---");
+            sb.AppendLine(FinalSnapshot);
+            return sb.ToString();
+        }
+    }
+
+    // ── Game Runner ───────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame()
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new WinCheckSystem());
+
+        for (int r = 0; r < 3; r++)
+            for (int c = 0; c < 3; c++)
+                world.AddComponent(world.CreateEntity(), new Cell { Row = r, Col = c });
+
+        world.SetSingleton(new GameState
+        {
+            CurrentPlayer = Player.X,
+            Status = GameStatus.Playing,
+            MoveCount = 0
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    private static void RunGame(World world, SystemGroup group, IAgent agentX, IAgent agentO, PlayLog log)
+    {
+        while (world.ReadSingleton<GameState>().Status == GameStatus.Playing)
+        {
+            var state = world.ReadSingleton<GameState>();
+            var agent = state.CurrentPlayer == Player.X ? agentX : agentO;
+            var cmd = agent.Decide(world);
+
+            world.Commands.Enqueue(cmd);
+            group.RunLogical();
+
+            log.Moves.Add($"{state.CurrentPlayer} → ({cmd.Row},{cmd.Col})");
+        }
+
+        var final = world.ReadSingleton<GameState>();
+        log.Header += $" — Result: {final.Status}";
+    }
+
+    // ── Snapshot ──────────────────────────────────────────────────────
+
+    private static string SnapshotWorld(World world)
+    {
+        var sb = new System.Text.StringBuilder();
+        var grid = new char?[3, 3];
+        using (var iter = world.Select<Cell, Mark>())
+        {
+            while (iter.MoveNext())
+                grid[iter.Current1.Row, iter.Current1.Col] =
+                    iter.Current2.Player == Player.X ? 'X' : 'O';
+        }
+
+        for (int r = 0; r < 3; r++)
+        {
+            sb.Append("  ");
+            for (int c = 0; c < 3; c++)
+                sb.Append(grid[r, c]?.ToString() ?? ".");
+            sb.AppendLine();
+        }
+        return sb.ToString().TrimEnd();
+    }
+
+    // ── File I/O ──────────────────────────────────────────────────────
+
+    private static string SavePlayLog(string filename, PlayLog log)
+    {
+        var dir = Path.Combine(AppContext.BaseDirectory, "playlogs");
+        Directory.CreateDirectory(dir);
+        var path = Path.Combine(dir, filename);
+        File.WriteAllText(path, log.Build());
+        return path;
+    }
+
+    private static void ReadAndVerify(string path)
+    {
+        var content = File.ReadAllText(path);
+
+        // Verify the play log is well-formed.
+        content.Should().Contain("--- Moves ---");
+        content.Should().Contain("--- Reactivity ---");
+        content.Should().Contain("--- Final Board ---");
+
+        // The game must have finished.
+        content.Should().Contain("Result:");
+        content.Should().NotContain("Result: Playing");
+    }
+
+    // ── Agents ────────────────────────────────────────────────────────
+
+    private interface IAgent
+    {
+        PlaceMarkCommand Decide(World world);
+    }
+
+    private sealed class RandomAgent : IAgent
+    {
+        private static readonly Random _rng = new();
+        public PlaceMarkCommand Decide(World world)
+        {
+            var empty = GetEmptyCells(world);
+            var (row, col) = empty[_rng.Next(empty.Count)];
+            return new PlaceMarkCommand { Row = row, Col = col };
+        }
+    }
+
+    private sealed class GreedyAgent : IAgent
+    {
+        public enum Strategy { WinOrBlock }
+        private readonly Strategy _strategy;
+        private static readonly Random _rng = new();
+        public GreedyAgent(Strategy strategy) => _strategy = strategy;
+
+        public PlaceMarkCommand Decide(World world)
+        {
+            var state = world.ReadSingleton<GameState>();
+            var me = state.CurrentPlayer;
+            var empty = GetEmptyCells(world);
+
+            foreach (var (r, c) in empty)
+                if (WouldWin(world, r, c, me))
+                    return new PlaceMarkCommand { Row = r, Col = c };
+
+            var opponent = me == Player.X ? Player.O : Player.X;
+            foreach (var (r, c) in empty)
+                if (WouldWin(world, r, c, opponent))
+                    return new PlaceMarkCommand { Row = r, Col = c };
+
+            if (empty.Any(c => c.Row == 1 && c.Col == 1))
+                return new PlaceMarkCommand { Row = 1, Col = 1 };
+
+            var (rr, cc) = empty[_rng.Next(empty.Count)];
+            return new PlaceMarkCommand { Row = rr, Col = cc };
+        }
+    }
+
+    private static List<(int Row, int Col)> GetEmptyCells(World world)
+    {
+        var empty = new List<(int, int)>();
+        var query = world.Query().With<Cell>().Without<Mark>().Build();
+        using var iter = world.Select<Cell>(query);
+        while (iter.MoveNext())
+            empty.Add((iter.Current1.Row, iter.Current1.Col));
+        return empty;
+    }
+
+    private static bool WouldWin(World world, int row, int col, Player player)
+    {
+        var grid = new Player[3, 3];
+        using (var iter = world.Select<Cell, Mark>())
+            while (iter.MoveNext())
+                grid[iter.Current1.Row, iter.Current1.Col] = iter.Current2.Player;
+        grid[row, col] = player;
+
+        for (int r = 0; r < 3; r++)
+            if (grid[r, 0] == player && grid[r, 1] == player && grid[r, 2] == player)
+                return true;
+        for (int c = 0; c < 3; c++)
+            if (grid[0, c] == player && grid[1, c] == player && grid[2, c] == player)
+                return true;
+        if (grid[0, 0] == player && grid[1, 1] == player && grid[2, 2] == player)
+            return true;
+        if (grid[0, 2] == player && grid[1, 1] == player && grid[2, 0] == player)
+            return true;
+
+        return false;
+    }
+}

Game.TicTacToe.Tests/SnapshotTests.cs

@@ -0,0 +1,244 @@
+using FluentAssertions;
+using Game.TicTacToe;
+using OECS;
+using R3;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Game.TicTacToe.Tests;
+
+/// <summary>
+/// Baseline snapshot and log tests for TicTacToe.
+/// Captures the world state as text and R3 change logs so they can be
+/// reviewed manually and compared across changes.
+/// </summary>
+public class SnapshotTests
+{
+    private readonly ITestOutputHelper _output;
+
+    public SnapshotTests(ITestOutputHelper output)
+    {
+        _output = output;
+    }
+
+    [Fact]
+    public void Snapshot_InitialBoard()
+    {
+        var world = new World();
+        for (int r = 0; r < 3; r++)
+            for (int c = 0; c < 3; c++)
+                world.AddComponent(world.CreateEntity(), new Cell { Row = r, Col = c });
+
+        world.SetSingleton(new GameState
+        {
+            CurrentPlayer = Player.X,
+            Status = GameStatus.Playing,
+            MoveCount = 0
+        });
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        // Verify baseline properties.
+        snapshot.Should().Contain("GameState: X's turn, 0 moves");
+        snapshot.Should().Contain("Cells: 9 total, 0 marked");
+    }
+
+    [Fact]
+    public void Snapshot_AfterThreeMoves()
+    {
+        var (world, group) = SetupGame();
+
+        // X: (0,0), O: (1,1), X: (2,2)
+        PlayMoves(world, group, (0, 0), (1, 1), (2, 2));
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("GameState: O's turn, 3 moves");
+        snapshot.Should().Contain("Cells: 9 total, 3 marked");
+        snapshot.Should().Contain("(0,0): X");
+        snapshot.Should().Contain("(1,1): O");
+        snapshot.Should().Contain("(2,2): X");
+    }
+
+    [Fact]
+    public void Snapshot_XWins()
+    {
+        var (world, group) = SetupGame();
+
+        // X: (0,0), O: (1,0), X: (0,1), O: (1,1), X: (0,2) → X wins row 0
+        PlayMoves(world, group, (0, 0), (1, 0), (0, 1), (1, 1), (0, 2));
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("GameState: XWon");
+        snapshot.Should().Contain("Cells: 9 total, 5 marked");
+    }
+
+    [Fact]
+    public void Snapshot_Draw()
+    {
+        var (world, group) = SetupGame();
+
+        PlayMoves(world, group,
+            (0, 0), (0, 1), (0, 2),
+            (1, 1), (1, 0), (1, 2),
+            (2, 1), (2, 0), (2, 2));
+
+        var snapshot = SnapshotWorld(world);
+        _output.WriteLine(snapshot);
+
+        snapshot.Should().Contain("GameState: Draw");
+        snapshot.Should().Contain("Cells: 9 total, 9 marked");
+    }
+
+    [Fact]
+    public void Log_ReactivityDuringGame()
+    {
+        var (world, group) = SetupGame();
+        var log = new List<string>();
+
+        // Subscribe to all entity-level changes.
+        world.ObserveEntityChanges().Subscribe(change =>
+        {
+            log.Add($"[entity] {change}");
+        });
+
+        // Subscribe to component-specific changes.
+        world.ObserveComponentChanges<Mark>().Subscribe(change =>
+        {
+            log.Add($"[mark] {change}");
+        });
+
+        world.ObserveComponentChanges<GameState>().Subscribe(change =>
+        {
+            log.Add($"[gamestate] {change}");
+        });
+
+        // Play a full game: X wins on row 0.
+        PlayMoves(world, group, (0, 0), (1, 0), (0, 1), (1, 1), (0, 2));
+
+        var logText = string.Join("\n", log);
+        _output.WriteLine(logText);
+
+        // Verify key events appear in the log.
+        log.Should().Contain(l => l.Contains("ComponentAdded") && l.Contains("Mark"));
+        log.Should().Contain(l => l.Contains("ComponentModified") && l.Contains("GameState"));
+    }
+
+    [Fact]
+    public void Serialization_RoundTrip_PreservesSnapshot()
+    {
+        var (world, group) = SetupGame();
+
+        // Play a few moves.
+        PlayMoves(world, group, (0, 0), (1, 1), (2, 2));
+
+        var before = SnapshotWorld(world);
+
+        using var stream = new MemoryStream();
+        WorldSerializer.Save(world, stream);
+        stream.Position = 0;
+
+        var world2 = new World();
+        WorldSerializer.Load(world2, stream);
+
+        var after = SnapshotWorld(world2);
+
+        _output.WriteLine("=== Before ===");
+        _output.WriteLine(before);
+        _output.WriteLine("=== After ===");
+        _output.WriteLine(after);
+
+        after.Should().Be(before);
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────
+
+    private static (World, SystemGroup) SetupGame()
+    {
+        var world = new World();
+        var group = new SystemGroup(world);
+        group.Add(new WinCheckSystem());
+
+        for (int r = 0; r < 3; r++)
+            for (int c = 0; c < 3; c++)
+                world.AddComponent(world.CreateEntity(), new Cell { Row = r, Col = c });
+
+        world.SetSingleton(new GameState
+        {
+            CurrentPlayer = Player.X,
+            Status = GameStatus.Playing,
+            MoveCount = 0
+        });
+        world.PostChanges();
+
+        return (world, group);
+    }
+
+    private static void PlayMoves(World world, SystemGroup group, params (int Row, int Col)[] moves)
+    {
+        foreach (var (row, col) in moves)
+        {
+            world.Commands.Enqueue(new PlaceMarkCommand { Row = row, Col = col });
+            group.RunLogical();
+        }
+    }
+
+    /// <summary>
+    /// Produces a human-readable textual snapshot of the world state.
+    /// </summary>
+    private static string SnapshotWorld(World world)
+    {
+        var sb = new System.Text.StringBuilder();
+
+        // Singleton: GameState.
+        var state = world.ReadSingleton<GameState>();
+        var statusText = state.Status switch
+        {
+            GameStatus.Playing => $"{(state.CurrentPlayer == Player.X ? 'X' : 'O')}'s turn, {state.MoveCount} moves",
+            GameStatus.XWon => "XWon",
+            GameStatus.OWon => "OWon",
+            GameStatus.Draw => "Draw",
+            _ => "Unknown"
+        };
+        sb.AppendLine($"GameState: {statusText}");
+
+        // Board: build a 3x3 grid.
+        var grid = new char?[3, 3];
+        using (var iter = world.Select<Cell, Mark>())
+        {
+            while (iter.MoveNext())
+            {
+                grid[iter.Current1.Row, iter.Current1.Col] =
+                    iter.Current2.Player == Player.X ? 'X' : 'O';
+            }
+        }
+
+        int totalCells = 0;
+        int markedCells = 0;
+        for (int r = 0; r < 3; r++)
+        {
+            for (int c = 0; c < 3; c++)
+            {
+                totalCells++;
+                var mark = grid[r, c];
+                if (mark.HasValue)
+                {
+                    markedCells++;
+                    sb.AppendLine($"  ({r},{c}): {mark.Value}");
+                }
+                else
+                {
+                    sb.AppendLine($"  ({r},{c}): .");
+                }
+            }
+        }
+
+        sb.AppendLine($"Cells: {totalCells} total, {markedCells} marked");
+
+        return sb.ToString().TrimEnd();
+    }
+}

Game.TicTacToe/Commands/PlaceMarkCommand.cs

@@ -1,7 +1,7 @@
 using MessagePack;
 using OECS;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// Places the current player's mark on the cell at (Row, Col).

Game.TicTacToe/Components/Cell.cs

@@ -1,12 +1,12 @@
 using MessagePack;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// Identifies a board position. One entity per cell.
 /// </summary>
 [MessagePackObject]
-public struct Cell
+public record struct Cell
 {
     [Key(0)] public int Row;
     [Key(1)] public int Col;

Game.TicTacToe/Components/Mark.cs

@@ -1,13 +1,13 @@
 using MessagePack;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// A mark placed on a cell by a player.
 /// Only present on cells that have been claimed.
 /// </summary>
 [MessagePackObject]
-public struct Mark
+public record struct Mark
 {
     [Key(0)] public Player Player;
 }

Game.TicTacToe/Components/Player.cs

@@ -1,4 +1,4 @@
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 public enum Player : byte
 {

Game.TicTacToe/CsvLoader.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// Minimal CSV loader that creates entities from a CSV file.

Game.TicTacToe/Singletons/GameState.cs

@@ -1,12 +1,12 @@
 using MessagePack;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// Global game state stored on the singleton entity.
 /// </summary>
 [MessagePackObject]
-public struct GameState
+public record struct GameState
 {
     [Key(0)] public Player CurrentPlayer;
     [Key(1)] public GameStatus Status;

Game.TicTacToe/Singletons/GameStatus.cs

@@ -1,4 +1,4 @@
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 public enum GameStatus : byte
 {

Game.TicTacToe/Systems/WinCheckSystem.cs

@@ -1,6 +1,6 @@
 using OECS;
 
-namespace TicTacToe;
+namespace Game.TicTacToe;
 
 /// <summary>
 /// After each move, checks whether the game has been won or drawn.

Game.TicTacToe/TicTacToe.csproj

@@ -1,19 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <OutputType>Exe</OutputType>
+    <OutputType>Library</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
+    <RootNamespace>Game.TicTacToe</RootNamespace>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-    <RootNamespace>TicTacToe</RootNamespace>
   </PropertyGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS\OECS.csproj" />
+    <ProjectReference Include="..\OECS\OECS.csproj" />
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS.SourceGen\OECS.SourceGen.csproj"
+    <ProjectReference Include="..\OECS.SourceGen\OECS.SourceGen.csproj"
                       OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
   </ItemGroup>
 

OECS.SourceGen/ComponentDiscoveryGenerator.cs

@@ -38,6 +38,10 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
         // Query building
         "With",
         "Without",
+
+        // Iteration and relationships
+        "Select",
+        "GetSources",
     };
 
     private static readonly HashSet<string> _forEachNames = new()
@@ -53,7 +57,7 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
                 predicate: IsCandidateInvocation,
                 transform: ExtractComponentType)
             .Where(t => t.FullyQualifiedName != null)
-            .Select((t, _) => (t.FullyQualifiedName!, t.AssemblyQualifiedName!));
+            .Select((t, _) => (t.FullyQualifiedName!, t.AssemblyQualifiedName!, t.IsRelationship));
 
         // Also collect ForEach type arguments from the World class.
         var forEachCalls = context.SyntaxProvider
@@ -100,7 +104,7 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
         return false;
     }
 
-    private static (string? FullyQualifiedName, string? AssemblyQualifiedName) ExtractComponentType(
+    private static (string? FullyQualifiedName, string? AssemblyQualifiedName, bool IsRelationship) ExtractComponentType(
         GeneratorSyntaxContext ctx, CancellationToken _)
     {
         if (ctx.Node is not InvocationExpressionSyntax invocation)
@@ -133,22 +137,25 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
             ? $"{typeSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat.WithGlobalNamespaceStyle(SymbolDisplayGlobalNamespaceStyle.Omitted))}, {typeSymbol.ContainingAssembly.Name}"
             : fqn;
 
-        return (fqn, aqn);
+        bool isRelationship = typeSymbol.AllInterfaces.Any(i =>
+            i.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat) == "global::OECS.IRelationship");
+
+        return (fqn, aqn, isRelationship);
     }
 
-    private static ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName)> ExtractForEachTypes(
+    private static ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName, bool IsRelationship)> ExtractForEachTypes(
         GeneratorSyntaxContext ctx, CancellationToken _)
     {
         if (ctx.Node is not InvocationExpressionSyntax invocation)
-            return ImmutableArray<(string, string)>.Empty;
+            return ImmutableArray<(string, string, bool)>.Empty;
 
         if (invocation.Expression is not MemberAccessExpressionSyntax memberAccess)
-            return ImmutableArray<(string, string)>.Empty;
+            return ImmutableArray<(string, string, bool)>.Empty;
 
         if (memberAccess.Name is not GenericNameSyntax genericName)
-            return ImmutableArray<(string, string)>.Empty;
+            return ImmutableArray<(string, string, bool)>.Empty;
 
-        var types = ImmutableArray.CreateBuilder<(string, string)>();
+        var types = ImmutableArray.CreateBuilder<(string, string, bool)>();
         foreach (var typeArg in genericName.TypeArgumentList.Arguments)
         {
             var symbolInfo = ctx.SemanticModel.GetSymbolInfo(typeArg);
@@ -160,7 +167,9 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
                 var aqn = typeSymbol.ContainingAssembly != null
                     ? $"{typeSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat.WithGlobalNamespaceStyle(SymbolDisplayGlobalNamespaceStyle.Omitted))}, {typeSymbol.ContainingAssembly.Name}"
                     : fqn;
-                types.Add((fqn, aqn));
+                bool isRelationship = typeSymbol.AllInterfaces.Any(i =>
+                    i.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat) == "global::OECS.IRelationship");
+                types.Add((fqn, aqn, isRelationship));
             }
         }
 
@@ -168,18 +177,21 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
     }
 
     private static void GenerateRegistry(SourceProductionContext context,
-        (ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName)> Left,
+        (ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName, bool IsRelationship)> Left,
-         ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName)> Right) data)
+         ImmutableArray<(string FullyQualifiedName, string AssemblyQualifiedName, bool IsRelationship)> Right) data)
     {
         var (invocations, forEachTypes) = data;
 
         // Collect unique types by fully-qualified name, deduplicating.
-        var typeMap = new Dictionary<string, (string Fqn, string Aqn)>();
+        var typeMap = new Dictionary<string, (string Fqn, string Aqn, bool IsRel)>();
         foreach (var t in invocations)
-            typeMap[t.FullyQualifiedName] = (t.FullyQualifiedName, t.AssemblyQualifiedName);
+            typeMap[t.FullyQualifiedName] = (t.FullyQualifiedName, t.AssemblyQualifiedName, t.IsRelationship);
         foreach (var t in forEachTypes)
         {
-            typeMap[t.FullyQualifiedName] = (t.FullyQualifiedName, t.AssemblyQualifiedName);
+            if (!typeMap.ContainsKey(t.FullyQualifiedName))
+                typeMap[t.FullyQualifiedName] = (t.FullyQualifiedName, t.AssemblyQualifiedName, t.IsRelationship);
+            else if (t.IsRelationship)
+                typeMap[t.FullyQualifiedName] = (t.FullyQualifiedName, t.AssemblyQualifiedName, true);
         }
 
         if (typeMap.Count == 0)
@@ -197,11 +209,11 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
         sb.AppendLine("        [ModuleInitializer]");
         sb.AppendLine("        public static void Initialize()");
         sb.AppendLine("        {");
-        sb.AppendLine("            ComponentRegistry.Initialize(new ComponentDescriptor[]");
+        sb.AppendLine("            ComponentRegistry.Register(new ComponentDescriptor[]");
         sb.AppendLine("            {");
 
         bool first = true;
-        foreach (var (fqn, aqn) in typeMap.Values.OrderBy(t => t.Fqn))
+        foreach (var (fqn, aqn, isRel) in typeMap.Values.OrderBy(t => t.Fqn))
         {
             if (!first)
                 sb.AppendLine(",");
@@ -212,7 +224,9 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
             sb.AppendLine($"                    type: typeof({fqn}),");
             sb.AppendLine($"                    serialize: obj => MessagePackSerializer.Serialize(({fqn})obj),");
             sb.AppendLine($"                    deserializeAndAdd: (world, entity, data) =>");
-            sb.AppendLine($"                        world.AddComponent(entity, MessagePackSerializer.Deserialize<{fqn}>(data))");
+            sb.AppendLine($"                        world.AddComponent(entity, MessagePackSerializer.Deserialize<{fqn}>(data)),");
+            sb.AppendLine($"                    deserialize: data => MessagePackSerializer.Deserialize<{fqn}>(data)");
+
             sb.Append("                )");
         }
 
@@ -224,4 +238,4 @@ public class ComponentDiscoveryGenerator : IIncrementalGenerator
 
         context.AddSource("ComponentRegistry.g.cs", sb.ToString());
     }
-}
+}

OECS.SourceGen/OECS.SourceGen.csproj

@@ -2,9 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-    <LangVersion>12</LangVersion>
     <RootNamespace>OECS.SourceGen</RootNamespace>
     <AssemblyName>OECS.SourceGen</AssemblyName>
     <EnforceExtendedAnalyzerRules>true</EnforceExtendedAnalyzerRules>

OECS.Tests/OECS.Tests.csproj

@@ -1,9 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <RootNamespace>OECS.Tests</RootNamespace>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
@@ -20,11 +17,11 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS\OECS.csproj" />
+    <ProjectReference Include="..\OECS\OECS.csproj" />
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS.SourceGen\OECS.SourceGen.csproj"
+    <ProjectReference Include="..\OECS.SourceGen\OECS.SourceGen.csproj"
                       OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
   </ItemGroup>
 

OECS.sln

@@ -1,19 +1,20 @@
-
+Microsoft Visual Studio Solution File, Format Version 12.00
-Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 17
 VisualStudioVersion = 17.0.31903.59
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS.SourceGen", "src\OECS.SourceGen\OECS.SourceGen.csproj", "{C3D4E5F6-A7B8-9012-CDEF-123456789012}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS", "OECS\OECS.csproj", "{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS", "src\OECS\OECS.csproj", "{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS.SourceGen", "OECS.SourceGen\OECS.SourceGen.csproj", "{C3D4E5F6-A7B8-9012-CDEF-123456789012}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS.Tests", "tests\OECS.Tests\OECS.Tests.csproj", "{B2C3D4E5-F6A7-8901-BCDE-F12345678901}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OECS.Tests", "OECS.Tests\OECS.Tests.csproj", "{B2C3D4E5-F6A7-8901-BCDE-F12345678901}"
 EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "examples", "examples", "{B36A84DF-456D-A817-6EDD-3EC3E7F6E11F}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Game.Blackjack", "Game.Blackjack\Blackjack.csproj", "{85631EDE-F984-4DA6-8EE9-0715AF1204E6}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Blackjack", "examples\Blackjack\Blackjack.csproj", "{85631EDE-F984-4DA6-8EE9-0715AF1204E6}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Game.TicTacToe", "Game.TicTacToe\TicTacToe.csproj", "{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}"
 EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{827E0CD3-B72D-47B6-A68D-7590B98EB39B}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Game.Blackjack.Tests", "Game.Blackjack.Tests\Game.Blackjack.Tests.csproj", "{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Game.TicTacToe.Tests", "Game.TicTacToe.Tests\Game.TicTacToe.Tests.csproj", "{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -25,18 +26,6 @@ Global
 		Release|x86 = Release|x86
 	EndGlobalSection
 	GlobalSection(ProjectConfigurationPlatforms) = postSolution
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x64.ActiveCfg = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x64.Build.0 = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x86.ActiveCfg = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x86.Build.0 = Debug|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|Any CPU.Build.0 = Release|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x64.ActiveCfg = Release|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x64.Build.0 = Release|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x86.ActiveCfg = Release|Any CPU
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x86.Build.0 = Release|Any CPU
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Debug|x64.ActiveCfg = Debug|Any CPU
@@ -49,6 +38,18 @@ Global
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Release|x64.Build.0 = Release|Any CPU
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Release|x86.ActiveCfg = Release|Any CPU
 		{A1B2C3D4-E5F6-7890-ABCD-EF1234567890}.Release|x86.Build.0 = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x64.Build.0 = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Debug|x86.Build.0 = Debug|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|Any CPU.Build.0 = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x64.ActiveCfg = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x64.Build.0 = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x86.ActiveCfg = Release|Any CPU
+		{C3D4E5F6-A7B8-9012-CDEF-123456789012}.Release|x86.Build.0 = Release|Any CPU
 		{B2C3D4E5-F6A7-8901-BCDE-F12345678901}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{B2C3D4E5-F6A7-8901-BCDE-F12345678901}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{B2C3D4E5-F6A7-8901-BCDE-F12345678901}.Debug|x64.ActiveCfg = Debug|Any CPU
@@ -73,12 +74,45 @@ Global
 		{85631EDE-F984-4DA6-8EE9-0715AF1204E6}.Release|x64.Build.0 = Release|Any CPU
 		{85631EDE-F984-4DA6-8EE9-0715AF1204E6}.Release|x86.ActiveCfg = Release|Any CPU
 		{85631EDE-F984-4DA6-8EE9-0715AF1204E6}.Release|x86.Build.0 = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|x64.Build.0 = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Debug|x86.Build.0 = Debug|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|Any CPU.Build.0 = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|x64.ActiveCfg = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|x64.Build.0 = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|x86.ActiveCfg = Release|Any CPU
+		{D4E5F6A7-B8C9-0123-DEF4-567890ABCDEF}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
-	GlobalSection(SolutionProperties) = preSolution
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|Any CPU.Build.0 = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|x64.ActiveCfg = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|x64.Build.0 = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|x86.ActiveCfg = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Debug|x86.Build.0 = Debug|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|Any CPU.ActiveCfg = Release|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|Any CPU.Build.0 = Release|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|x64.ActiveCfg = Release|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|x64.Build.0 = Release|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|x86.ActiveCfg = Release|Any CPU
+			{E5F6A7B8-C9D0-1234-EF56-7890ABCDEF01}.Release|x86.Build.0 = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|Any CPU.Build.0 = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|x64.ActiveCfg = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|x64.Build.0 = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|x86.ActiveCfg = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Debug|x86.Build.0 = Debug|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|Any CPU.ActiveCfg = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|Any CPU.Build.0 = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|x64.ActiveCfg = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|x64.Build.0 = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|x86.ActiveCfg = Release|Any CPU
+			{F6A7B8C9-D0E1-2345-F678-90ABCDEF0123}.Release|x86.Build.0 = Release|Any CPU
+		EndGlobalSection
+		GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
-	GlobalSection(NestedProjects) = preSolution
-		{C3D4E5F6-A7B8-9012-CDEF-123456789012} = {827E0CD3-B72D-47B6-A68D-7590B98EB39B}
-		{85631EDE-F984-4DA6-8EE9-0715AF1204E6} = {B36A84DF-456D-A817-6EDD-3EC3E7F6E11F}
-	EndGlobalSection
 EndGlobal

OECS/ComponentDescriptor.cs

@@ -28,15 +28,23 @@ public sealed class ComponentDescriptor
     /// </summary>
     public Action<World, Entity, byte[]> DeserializeAndAdd { get; }
 
+    /// <summary>
+    /// Deserializes a MessagePack byte array to a boxed component object.
+    /// Used by <see cref="WorldSerializer"/> for relationship fixup before adding.
+    /// </summary>
+    public Func<byte[], object> Deserialize { get; }
+
     public ComponentDescriptor(
         string typeName,
         Type type,
         Func<object, byte[]> serialize,
-        Action<World, Entity, byte[]> deserializeAndAdd)
+        Action<World, Entity, byte[]> deserializeAndAdd,
+        Func<byte[], object> deserialize)
     {
         TypeName = typeName;
         Type = type;
         Serialize = serialize;
         DeserializeAndAdd = deserializeAndAdd;
+        Deserialize = deserialize;
     }
 }

OECS/ComponentRegistry.cs

@@ -0,0 +1,136 @@
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using MessagePack;
+
+namespace OECS;
+
+/// <summary>
+/// Registry of component types used by <see cref="WorldSerializer"/>.
+/// Types are discovered via the OECS.SourceGen incremental generator
+/// at compile time, with a runtime fallback that scans loaded assemblies
+/// for <c>[MessagePackObject]</c> structs.
+/// </summary>
+public static class ComponentRegistry
+{
+    private static ComponentDescriptor[]? _descriptors;
+    private static Dictionary<string, ComponentDescriptor>? _byTypeName;
+    private static bool _scanned;
+
+    /// <summary>
+    /// All discovered component descriptors.
+    /// </summary>
+    public static ComponentDescriptor[] Descriptors
+    {
+        get
+        {
+            EnsureScanned();
+            return _descriptors ?? Array.Empty<ComponentDescriptor>();
+        }
+    }
+
+    /// <summary>
+    /// Lookup by assembly-qualified type name.
+    /// </summary>
+    public static IReadOnlyDictionary<string, ComponentDescriptor> ByTypeName
+    {
+        get
+        {
+            EnsureScanned();
+            if (_byTypeName == null)
+            {
+                var dict = new Dictionary<string, ComponentDescriptor>();
+                foreach (var desc in Descriptors)
+                    dict[desc.TypeName] = desc;
+                _byTypeName = dict;
+            }
+            return _byTypeName;
+        }
+    }
+
+    /// <summary>
+    /// Called by generated code to register discovered component types.
+    /// Multiple assemblies may call this. Descriptors are accumulated.
+    /// </summary>
+    public static void Register(ComponentDescriptor[] descriptors)
+    {
+        if (_descriptors == null)
+        {
+            _descriptors = descriptors;
+        }
+        else
+        {
+            var existing = new HashSet<string>(_descriptors.Select(d => d.TypeName));
+            var merged = new List<ComponentDescriptor>(_descriptors);
+            foreach (var d in descriptors)
+            {
+                if (!existing.Contains(d.TypeName))
+                {
+                    existing.Add(d.TypeName);
+                    merged.Add(d);
+                }
+            }
+            _descriptors = merged.ToArray();
+        }
+        _byTypeName = null;
+    }
+
+    private static void EnsureScanned()
+    {
+        if (_scanned) return;
+        _scanned = true;
+
+        // Runtime fallback: scan loaded assemblies for [MessagePackObject] structs.
+        // This ensures serialization works even when the source generator
+        // doesn't run (e.g., in test projects referencing game DLLs).
+        var scanned = new List<ComponentDescriptor>(_descriptors ?? Array.Empty<ComponentDescriptor>());
+        var seen = new HashSet<string>(scanned.Select(d => d.TypeName));
+
+        foreach (var asm in AppDomain.CurrentDomain.GetAssemblies())
+        {
+            try
+            {
+                foreach (var type in asm.GetTypes())
+                {
+                    if (!type.IsValueType || type.IsAbstract || !type.IsPublic)
+                        continue;
+                    if (type.GetCustomAttribute<MessagePackObjectAttribute>() == null)
+                        continue;
+
+                    var aqn = $"{type.FullName}, {type.Assembly.GetName().Name}";
+                    if (!seen.Add(aqn))
+                        continue;
+
+                    var desc = CreateDescriptor(type, aqn);
+                    scanned.Add(desc);
+                }
+            }
+            catch
+            {
+                // Some assemblies may throw during reflection (e.g., mixed-mode).
+            }
+        }
+
+        _descriptors = scanned.ToArray();
+    }
+
+    private static ComponentDescriptor CreateDescriptor(Type type, string aqn)
+    {
+        // Build serialize/deserialize delegates via reflection.
+        var method = typeof(ComponentRegistry).GetMethod(
+            nameof(CreateTypedDescriptor),
+            BindingFlags.NonPublic | BindingFlags.Static)!;
+        var generic = method.MakeGenericMethod(type);
+        return (ComponentDescriptor)generic.Invoke(null, [aqn])!;
+    }
+
+    private static ComponentDescriptor CreateTypedDescriptor<T>(string aqn) where T : struct
+    {
+        return new ComponentDescriptor(
+            typeName: aqn,
+            type: typeof(T),
+            serialize: obj => MessagePackSerializer.Serialize((T)obj),
+            deserializeAndAdd: (world, entity, data) =>
+                world.AddComponent(entity, MessagePackSerializer.Deserialize<T>(data)),
+            deserialize: data => MessagePackSerializer.Deserialize<T>(data));
+    }
+}

OECS/ComponentStore.cs

@@ -109,21 +109,4 @@ internal class ComponentStore
         return set;
     }
 
-    /// <summary>
-    /// Returns the count of entities in the sparse set for type <typeparamref name="T"/>.
-    /// Returns 0 if the set does not exist.
-    /// </summary>
-    public int Count<T>() where T : struct
-    {
-        return GetSetIfExists<T>()?.Count ?? 0;
-    }
-
-    /// <summary>
-    /// Returns the count of entities in the sparse set for the given type.
-    /// Returns 0 if the set does not exist.
-    /// </summary>
-    public int Count(Type componentType)
-    {
-        return GetSet(componentType)?.Count ?? 0;
-    }
 }

OECS/OECS.csproj

@@ -1,9 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <RootNamespace>OECS</RootNamespace>
     <AssemblyName>OECS</AssemblyName>
 

OECS/RelationshipIndex.cs

@@ -70,24 +70,6 @@ internal class RelationshipIndex
         return sources;
     }
 
-    /// <summary>
-    /// Returns all target entities that the given source entity points to
-    /// via the specified relationship type. Returns an empty collection if none.
-    /// </summary>
-    public IReadOnlyCollection<Entity> GetTargets(Type relationshipType, Entity source)
-    {
-        if (!_index.TryGetValue(relationshipType, out var targetMap))
-            return Array.Empty<Entity>();
-
-        var result = new List<Entity>();
-        foreach (var (target, sources) in targetMap)
-        {
-            if (sources.Contains(source))
-                result.Add(target);
-        }
-        return result;
-    }
-
     /// <summary>
     /// Removes all index entries where the given entity appears as a source.
     /// Called before the entity's components are removed during destruction.

OECS/SparseSet.cs

@@ -132,29 +132,6 @@ internal class SparseSet<T> : ISparseSet where T : struct
         return ref _dense[index];
     }
 
-    /// <summary>
-    /// Tries to get the component value for the given entity.
-    /// Returns true and copies the value to <paramref name="value"/> if present.
-    /// </summary>
-    public bool TryGet(Entity entity, out T value)
-    {
-        if (entity.Id >= (uint)_sparse.Length)
-        {
-            value = default;
-            return false;
-        }
-
-        int index = _sparse[entity.Id];
-        if (index == -1)
-        {
-            value = default;
-            return false;
-        }
-
-        value = _dense[index];
-        return true;
-    }
-
     private static void ThrowEntityNotPresent(Entity entity)
     {
         throw new InvalidOperationException(

OECS/World.cs

@@ -181,6 +181,23 @@ public class World : IDisposable
         AddComponentImpl(entity, component);
     }
 
+    /// <summary>
+    /// Adds a boxed component to the entity. Used by <see cref="WorldSerializer"/>
+    /// during deserialization after relationship Source fixup.
+    /// Calls <see cref="AddComponentImpl{T}"/> via reflection to avoid
+    /// duplicating the relationship index logic.
+    /// </summary>
+    internal void AddComponentBoxed(Entity entity, object component, Type componentType)
+    {
+        var method = s_addComponentImpl.MakeGenericMethod(componentType);
+        method.Invoke(this, [entity, component]);
+    }
+
+    private static readonly System.Reflection.MethodInfo s_addComponentImpl =
+        typeof(World).GetMethod(
+            nameof(AddComponentImpl),
+            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!;
+
     private void AddComponentImpl<T>(Entity entity, T component) where T : struct
     {
         ThrowIfNotAlive(entity);

OECS/WorldSerializer.cs

@@ -1,3 +1,4 @@
+using System.Reflection;
 using MessagePack;
 
 namespace OECS;
@@ -93,9 +94,32 @@ public static class WorldSerializer
                         $"Ensure the component type is used with the World " +
                         $"API so the source generator can discover it.");
 
-                // Deserialize and add in one typed operation — no reflection.
+                // Deserialize and fix up IRelationship.Source before adding.
-                desc.DeserializeAndAdd(world, entity, ce.Data);
+                var component = desc.Deserialize(ce.Data);
+                if (component is IRelationship rel)
+                    FixupRelationshipSource(rel, entity);
+
+                // Add via the typed internal method — no reflection for the add itself.
+                world.AddComponentBoxed(entity, component, desc.Type);
             }
         }
     }
+
+    /// <summary>
+    /// Sets the Source field on a deserialized IRelationship to match
+    /// the entity it's being restored to. Uses a small cache of PropertyInfo
+    /// to avoid repeated reflection lookups.
+    /// </summary>
+    private static void FixupRelationshipSource(IRelationship rel, Entity entity)
+    {
+        var type = rel.GetType();
+        if (!_sourcePropCache.TryGetValue(type, out var prop))
+        {
+            prop = type.GetProperty("Source");
+            _sourcePropCache[type] = prop;
+        }
+        prop?.SetValue(rel, entity);
+    }
+
+    private static readonly Dictionary<Type, PropertyInfo?> _sourcePropCache = new();
 }

docs/api-surface.md

@@ -139,13 +139,17 @@ public delegate void ForEachAction<T1, T2, T3, T4, T5, T6>(Entity entity, ref T1
 
 Extension methods on `World` for `foreach`-style iteration over queries.
 Returns `ref struct` iterators that support zero-allocation iteration with
-`ref` access to components. Supports 1–3 component types.
+`ref` access to components.
+
+Two sets of overloads: one accepting an explicit `QueryDescriptor` for
+filtering with `Without<T>`, and one without for simple "has component" scans.
 
 ```csharp
 namespace OECS;
 
 public static class EntityIterator
 {
+    // With explicit QueryDescriptor (supports Without<T> filters).
     public static Select1<T1> Select<T1>(
         this World world, QueryDescriptor query) where T1 : struct;
 
@@ -156,16 +160,38 @@ public static class EntityIterator
     public static Select3<T1, T2, T3> Select<T1, T2, T3>(
         this World world, QueryDescriptor query)
         where T1 : struct where T2 : struct where T3 : struct;
+
+    // Without QueryDescriptor — iterates all entities with the given component(s).
+    public static Select1<T1> Select<T1>(
+        this World world) where T1 : struct;
+
+    public static Select2<T1, T2> Select<T1, T2>(
+        this World world)
+        where T1 : struct where T2 : struct;
+
+    public static Select3<T1, T2, T3> Select<T1, T2, T3>(
+        this World world)
+        where T1 : struct where T2 : struct where T3 : struct;
 }
 ```
 
-Usage:
+The singletons `CurrentEntity`, `Current1`, `Current2`, `Current3` are
+exposed on the ref struct iterators directly. Usage:
 
 ```csharp
-using var iter = world.Select<Position, Velocity>(query);
+// Simple scan: all entities with PlayerHand.
-foreach (ref var item in iter)
+using var iter = world.Select<PlayerHand>();
+while (iter.MoveNext())
 {
-    item.Current1.X += item.Current2.X * dt;
+    Console.WriteLine(iter.CurrentEntity);
+}
+
+// With query filter:
+var query = world.Query().With<Position>().Without<Frozen>().Build();
+using var iter2 = world.Select<Position>(query);
+while (iter2.MoveNext())
+{
+    iter2.Current1.X += 1;
 }
 ```
 
@@ -202,20 +228,32 @@ public class QueryDescriptor
 
 ## ISystem
 
+A system is a unit of logic that runs during a tick. It receives the `World`
+and decides what to do — typically reading singletons, iterating queries,
+or enqueuing commands.
+
 ```csharp
 namespace OECS;
 
 public interface ISystem
 {
-    QueryDescriptor Query { get; }
     void Run(World world);
 }
 ```
 
+Systems do **not** declare a query on the interface. Instead, they either
+read singletons directly (`world.ReadSingleton<T>()`) or use the iterator /
+`ForEach` API with a `QueryDescriptor` they build internally. This keeps
+the interface minimal and gives systems full flexibility over what they
+inspect at runtime.
+
 ---
 
 ## ITickedSystem
 
+An optional extension for systems that need tick metadata (delta time or
+logical tick marker).
+
 ```csharp
 namespace OECS;
 
@@ -225,6 +263,10 @@ public interface ITickedSystem : ISystem
 }
 ```
 
+`SystemGroup` checks each system at runtime: if it implements
+`ITickedSystem`, the `Run(World, Tick)` overload is called; otherwise
+`Run(World)` is called. Both overloads must be implemented.
+
 ---
 
 ## Tick
@@ -266,6 +308,9 @@ public class SystemGroup
 }
 ```
 
+Systems execute in registration order. `SystemGroup` automatically drains
+commands and posts changes after each system and after the full tick.
+
 ---
 
 ## ICommand
@@ -279,6 +324,9 @@ public interface ICommand
 }
 ```
 
+Implementations should be `[MessagePackObject]` structs so they can be
+serialized and replayed.
+
 ---
 
 ## CommandQueue
@@ -297,6 +345,9 @@ public class CommandQueue
 }
 ```
 
+`Enqueue<T>` uses a constrained generic to avoid boxing at the call site.
+Commands enqueued during `ExecuteAll` are processed in the same drain cycle.
+
 ---
 
 ## IRelationship
@@ -324,6 +375,7 @@ namespace OECS;
 
 [MessagePackObject]
 public struct Relationship<TSelf, TTarget> : IRelationship
+    where TSelf : struct where TTarget : struct
 {
     [Key(0)] public Entity Source { get; set; }
     [Key(1)] public Entity Target { get; set; }
@@ -359,7 +411,7 @@ public enum ChangeKind
 ## WorldSerializer
 
 Saves and loads a `World` to/from a MessagePack stream. Components are
-serialized by their runtime type.
+serialized by their runtime type using the source-generated `ComponentRegistry`.
 
 ```csharp
 namespace OECS;
@@ -393,6 +445,7 @@ public class EntitySnapshot
     [Key(0)] public uint Id { get; set; }
     [Key(1)] public uint Version { get; set; }
     [Key(2)] public ComponentEntry[] Components { get; set; }
+    [IgnoreMember]
     public Entity Entity { get; }
 }
 
@@ -410,37 +463,30 @@ public class ComponentEntry
 
 ```csharp
 using OECS;
-using R3;
 
 // Define components
 [MessagePackObject]
-public struct Position : IMessagePackSerializationCallbackReceiver
+public struct Position
 {
     [Key(0)] public float X;
     [Key(1)] public float Y;
-
-    public void OnBeforeSerialize() { }
-    public void OnAfterDeserialize() { }
 }
 
 [MessagePackObject]
-public struct Velocity : IMessagePackSerializationCallbackReceiver
+public struct Velocity
 {
     [Key(0)] public float X;
     [Key(1)] public float Y;
-
-    public void OnBeforeSerialize() { }
-    public void OnAfterDeserialize() { }
 }
 
 // Define a system
 public class MovementSystem : ITickedSystem
 {
-    public QueryDescriptor Query { get; }
+    private readonly QueryDescriptor _query;
 
     public MovementSystem(World world)
     {
-        Query = world.Query()
+        _query = world.Query()
             .With<Position>()
             .With<Velocity>()
             .Build();
@@ -452,7 +498,7 @@ public class MovementSystem : ITickedSystem
     {
         float dt = tick.DeltaTime;
 
-        world.ForEach(Query, (Entity entity, ref Position pos, ref Velocity vel) =>
+        world.ForEach(_query, (Entity entity, ref Position pos, ref Velocity vel) =>
         {
             pos.X += vel.X * dt;
             pos.Y += vel.Y * dt;
@@ -466,16 +512,15 @@ var world = new World();
 var group = new SystemGroup(world);
 group.Add(new MovementSystem(world));
 
-// Observe changes
-world.ObserveComponentChanges<Position>()
-    .Subscribe(change => Console.WriteLine($"{change.Entity} moved"))
-    .AddTo(disposables);
-
 // Create entities
 var player = world.CreateEntity();
 world.AddComponent(player, new Position { X = 0, Y = 0 });
 world.AddComponent(player, new Velocity { X = 1, Y = 0 });
 
+// Observe changes via R3
+world.ObserveComponentChanges<Position>()
+    .Subscribe(change => Console.WriteLine($"{change.Entity} moved"));
+
 // Run a tick
 group.RunTimed(0.016f); // ~60 FPS
 ```
@@ -494,4 +539,6 @@ These types are implementation details and may change without notice:
 | `ChangeSet` | Deduplicated change accumulator within a single batch. |
 | `RelationshipIndex` | Reverse lookup from target entity to source entities. |
 | `EntityAllocator` | Free-list + bump allocator for entity IDs. |
-| `QueryExecutor` | Query iteration logic with smallest-set driver optimization. |
+| `QueryExecutor` | Query iteration logic with smallest-set driver optimization. |
+| `ComponentRegistry` | Source-generated registry of all component types for serialization. |
+| `ComponentDescriptor` | Source-generated per-type descriptor with serialize/deserialize callbacks. |

docs/implementation-plan.md

@@ -8,9 +8,10 @@ ones. Architecture decisions are captured in `docs/architecture.md`.
 
 ## Target
 
-- .NET 8 (LTS), `net8.0`
+- .NET 8 (LTS), `net8.0` (C# 12)
 - Dependencies: `MessagePack` (serialization), `R3` (reactivity)
 - Output: `OECS.dll`
+- Source generator: `OECS.SourceGen.dll` (component registry for serialization)
 
 ---
 
@@ -22,8 +23,12 @@ ones. Architecture decisions are captured in `docs/architecture.md`.
 
 ```
 OECS.sln
-├── src/OECS/OECS.csproj
+├── OECS/OECS.csproj
-└── tests/OECS.Tests/OECS.Tests.csproj
+├── OECS.SourceGen/OECS.SourceGen.csproj
+├── OECS.Tests/OECS.Tests.csproj
+├── Game.Blackjack/Blackjack.csproj
+├── Game.Blackjack.Tests/Game.Blackjack.Tests.csproj
+└── Game.TicTacToe/...
 ```
 
 - `OECS.csproj` targets `net8.0`, references `MessagePack` and `R3`.
@@ -76,6 +81,8 @@ Holds a `Dictionary<Type, object>` mapping component types to their
 - `void Add<T>(Entity, T)`
 - `void Remove<T>(Entity)`
 - `ref T Get<T>(Entity)`
+- `T Read<T>(Entity)` — copy without auto-mark
+- `bool TryGet<T>(Entity, out T)`
 - `bool Has<T>(Entity)`
 - `void RemoveAll(Entity)` — called on entity destruction
 
@@ -99,6 +106,8 @@ void DestroyEntity(Entity entity);
 void AddComponent<T>(Entity entity, T component) where T : struct;
 void RemoveComponent<T>(Entity entity) where T : struct;
 ref T GetComponent<T>(Entity entity) where T : struct;
+T ReadComponent<T>(Entity entity) where T : struct;
+bool TryGetComponent<T>(Entity entity, out T value) where T : struct;
 bool HasComponent<T>(Entity entity) where T : struct;
 bool IsAlive(Entity entity);
 ```
@@ -128,8 +137,8 @@ A query is defined by:
 ```
 class QueryDescriptor
 {
-    HashSet<Type> With { get; }
+    IReadOnlySet<Type> With { get; }
-    HashSet<Type> Without { get; }
+    IReadOnlySet<Type> Without { get; }
 }
 ```
 
@@ -147,49 +156,74 @@ world.Query()
 
 ### 2.3 Query Execution
 
-`World` provides iteration over matching entities. The smallest "with" sparse
+`World` provides two iteration styles:
-set is used as the driver; other sets are probed for membership.
 
+**ForEach callbacks** (1–6 component types):
 ```csharp
-void ForEach<T1, T2>(QueryDescriptor query, Action<Entity, ref T1, ref T2> action);
+void ForEach<T1, T2>(QueryDescriptor query, ForEachAction<T1, T2> action);
 ```
 
-Overloads for 1–6 component types. The `Without` filter is checked by probing
+**Ref struct iterators** via `EntityIterator.Select<T>()` extensions (1–3 component types):
-the corresponding sparse sets.
+```csharp
+using var iter = world.Select<Position, Velocity>(query);
+while (iter.MoveNext()) { ... }
+```
+
+Both accept an optional `QueryDescriptor`. When omitted, all entities with
+the given component types are iterated (no `Without` filter).
+
+The smallest "with" sparse set is used as the driver; other sets are probed
+for membership. The singleton entity (ID 1) is always skipped.
 
 ### 2.4 ISystem
 
 ```
 interface ISystem
 {
-    QueryDescriptor Query { get; }
     void Run(World world);
 }
 ```
 
-Systems declare their query and receive the world in `Run`. They call
+Systems do **not** declare a query on the interface. Instead, they either
-`world.ForEach(query, ...)` to iterate.
+read singletons directly or build queries internally and iterate with
+`ForEach` / `Select`. This keeps the interface minimal and gives systems
+full flexibility.
 
-Alternative considered: auto-injection of component refs. Rejected because it
+**Why no auto-injection?** The explicit API makes the iteration cost
-hides the iteration cost and makes the API less explicit. The explicit
+visible and avoids the need for source generators or reflection for
-`ForEach` call keeps the system author aware of what they're iterating.
+system dispatch.
 
-### 2.5 System Registration & Ordering
+### 2.5 ITickedSystem
+
+```
+interface ITickedSystem : ISystem
+{
+    void Run(World world, Tick tick);
+}
+```
+
+Systems that need tick metadata (delta time or logical tick marker)
+implement this. Both `Run` overloads must be implemented; `SystemGroup`
+dispatches to the appropriate one at runtime.
+
+### 2.6 System Registration & Ordering
 
 ```
 class SystemGroup
 {
     void Add(ISystem system);          // registration order = execution order
-    void RunTimed(float deltaTime);    // calls each system's Run
+    void Remove(ISystem system);
-    void RunLogical();                 // calls each system's Run
+    void RunTimed(float deltaTime);
+    void RunLogical();
+    int Count { get; }
 }
 ```
 
-Systems run in registration order. For now, no explicit dependency graph —
+Systems run in registration order. `SystemGroup` automatically:
-this is the simplest model that works. If needed later, `Before()`/`After()`
+- Drains commands before the tick, after each system, and after the full tick.
-constraints can be added without breaking the API.
+- Posts changes after each system and after the full tick.
 
-### 2.6 Tick
+### 2.7 Tick
 
 ```
 readonly struct Tick
@@ -199,16 +233,7 @@ readonly struct Tick
 }
 ```
 
-Passed to systems that opt into it via a separate interface:
+### 2.8 Tests
-
-```
-interface ITickedSystem : ISystem
-{
-    void Run(World world, Tick tick);
-}
-```
-
-### 2.7 Tests
 
 - Query with single component returns matching entities.
 - Query with multiple components returns intersection.
@@ -216,6 +241,7 @@ interface ITickedSystem : ISystem
 - Adding/removing components updates query results.
 - Systems run in registration order.
 - Destroyed entities don't appear in queries.
+- `ITickedSystem` receives correct tick data.
 
 ---
 
@@ -226,31 +252,38 @@ interface ITickedSystem : ISystem
 ### 3.1 ICommand
 
 ```
-[MessagePackObject]
 interface ICommand
 {
     void Execute(World world);
 }
 ```
 
-Commands are `[MessagePackObject]` structs implementing `ICommand`. They are
+Commands are structs implementing `ICommand`. They are typically
-not stored in ECS sparse sets — they live in a queue.
+`[MessagePackObject]` for serialization. Not stored in ECS sparse
+sets — they live in a queue.
 
 ### 3.2 CommandQueue
 
 ```
 class CommandQueue
 {
-    void Enqueue(ICommand command);
+    void Enqueue<T>(T command) where T : struct, ICommand;
-    void ExecuteAll(World world);  // FIFO, clears queue
+    void ExecuteAll(World world);  // FIFO, fully drains queue
+    void Clear();
+    void ClearErrors();
     int Count { get; }
+    IReadOnlyList<Exception> Errors { get; }
 }
 ```
 
+`Enqueue<T>` uses a constrained generic to avoid boxing at the call site.
+Commands enqueued during `ExecuteAll` are processed in the same drain cycle.
+
 ### 3.3 Integration with World
 
 `World` owns a `CommandQueue`. After each system runs (or after the full tick),
-the queue is drained. This is configurable:
+the queue is drained automatically by `SystemGroup`. Manual drain is also
+available:
 
 ```csharp
 world.ExecuteCommands(); // manual drain
@@ -279,36 +312,48 @@ available via `CommandQueue.Errors`.
 **Goal:** Relationship components with auto-managed source/target and reverse
 lookup.
 
-### 4.1 Relationship\<TSelf, TTarget\>
+### 4.1 IRelationship
+
+```
+interface IRelationship
+{
+    Entity Source { get; }
+    Entity Target { get; }
+}
+```
+
+### 4.2 Relationship\<TSelf, TTarget\>
 
 ```
 [MessagePackObject]
 struct Relationship<TSelf, TTarget> : IRelationship
+    where TSelf : struct where TTarget : struct
 {
     [Key(0)] Entity Source { get; set; }
     [Key(1)] Entity Target { get; set; }
-    // ... payload fields
 }
 ```
 
 The `IRelationship` marker interface lets `ComponentStore` detect relationships
-and maintain the reverse index.
+and maintain the reverse index. The `TSelf`/`TTarget` phantom types
+differentiate relationship kinds at the type level.
 
-### 4.2 Reverse Index
+Alternatively, implement `IRelationship` directly on your own struct (see the
+Blackjack `Holds` and `InDeck` types).
 
-`World` maintains a `Dictionary<Entity, HashSet<Entity>>` per relationship
+### 4.3 Reverse Index
-type, mapping target → set of source entities.
 
-When a relationship component is added/removed, the index is updated
+`World` maintains a reverse index per relationship type, mapping target →
-automatically.
+set of source entities. Updated automatically when relationship components
+are added/removed.
 
-### 4.3 Reverse Lookup API
+### 4.4 Reverse Lookup API
 
 ```csharp
 IReadOnlyCollection<Entity> GetSources<T>(Entity target) where T : struct, IRelationship;
 ```
 
-### 4.4 Cascading Behavior
+### 4.5 Cascading Behavior
 
 When an entity is destroyed:
 - All relationships where it is the **source** are removed (components dropped).
@@ -316,7 +361,7 @@ When an entity is destroyed:
   from source entities).
 - The reverse index is cleaned up.
 
-### 4.5 Tests
+### 4.6 Tests
 
 - Adding a relationship updates the reverse index.
 - Removing a relationship updates the reverse index.
@@ -346,23 +391,17 @@ struct EntityChange
 {
     Entity Entity { get; }
     ChangeKind Kind { get; }
-    Type ComponentType { get; } // null for entity-level changes
+    Type? ComponentType { get; } // null for entity-level changes
 }
 ```
 
-### 5.2 ChangeSet
+### 5.2 ChangeBuffer
 
 ```
-class ChangeSet
+class ChangeBuffer
 {
-    void MarkEntityAdded(Entity entity);
+    void Mark<...>(...);
-    void MarkEntityRemoved(Entity entity);
+    void Post();  // pushes to R3 subjects, then clears
-    void MarkComponentAdded(Entity entity, Type componentType);
-    void MarkComponentRemoved(Entity entity, Type componentType);
-    void MarkComponentModified(Entity entity, Type componentType);
-
-    IReadOnlyList<EntityChange> Changes { get; }
-    void Clear();
 }
 ```
 
@@ -381,25 +420,15 @@ Rationale: structural changes are always detectable. Value mutations inside a
 value. Requiring an explicit `MarkModified` call is the simplest correct
 approach.
 
-**Debug aid:** In `DEBUG` builds, `ref T Get<T>(Entity)` returns a wrapper that
+**`ReadComponent<T>`** (returns a copy) and **`ReadSingleton<T>`** never
-tracks whether the value was written. If a system iterates `ref T` and never
+auto-mark. Use these when you only need to inspect state without signaling
-calls `MarkModified`, a warning is logged. This catches the most common
+changes.
-mistake.
 
 ### 5.4 Posting Model
 
-```
+- During a system's `Run`, changes accumulate in a pending buffer.
-class ChangeBuffer
+- After each system's `Run`, `Post()` is called automatically by `SystemGroup`.
-{
+- After the full tick, `Post()` is called once more.
-    ChangeSet Pending { get; }    // accumulates during system run
-    void Post();                  // pushes to R3 subjects, then clears
-}
-```
-
-- During a system's `Run`, changes accumulate in `Pending`.
-- After each system's `Run`, `Post()` is called automatically.
-- After the full tick, `Post()` is called once more (for any changes made
-  outside systems, e.g., during command execution).
 - When not in a system run, changes are **not** posted automatically — the
   caller must call `world.PostChanges()`.
 
@@ -408,9 +437,9 @@ class ChangeBuffer
 `World` exposes observables:
 
 ```csharp
-IObservable<EntityChange> ObserveEntityChanges();
+Observable<EntityChange> ObserveEntityChanges();
-IObservable<EntityChange> ObserveComponentChanges<T>() where T : struct;
+Observable<EntityChange> ObserveComponentChanges<T>() where T : struct;
-IObservable<EntityChange> ObserveQuery(QueryDescriptor query);
+Observable<EntityChange> ObserveQuery(QueryDescriptor query);
 ```
 
 These are backed by `Subject<EntityChange>` instances. Subscribers receive
@@ -445,19 +474,21 @@ world.ObserveComponentChanges<Health>()
 ### 6.1 Singleton Entity
 
 `World` reserves entity ID `1` as the singleton entity. It is never destroyed
-and is excluded from normal queries by default.
+and is excluded from normal queries (all iterators skip ID 1).
 
 ### 6.2 Singleton Accessors
 
 ```csharp
 void SetSingleton<T>(T component) where T : struct;
 ref T GetSingleton<T>() where T : struct;
+T ReadSingleton<T>() where T : struct;
 bool HasSingleton<T>() where T : struct;
 void RemoveSingleton<T>() where T : struct;
 ```
 
 These are convenience wrappers around `AddComponent`/`GetComponent` on the
-singleton entity.
+singleton entity. `GetSingleton` returns a `ref` (auto-marks during iteration);
+`ReadSingleton` returns a copy (never auto-marks).
 
 ### 6.3 Query Exclusion
 
@@ -473,28 +504,62 @@ to include it, they can query it by its entity ID directly.
 
 ---
 
-## Phase 7 — Polish & Documentation (Week 7)
+## Phase 7 — Source Generator & Serialization (Week 7)
 
-### 7.1 XML Docs
+**Goal:** Compile-time component registry for serialization without reflection.
+
+### 7.1 OECS.SourceGen
+
+A Roslyn incremental source generator that scans for all types used as
+generic arguments to `World` methods (`AddComponent<T>`, `GetComponent<T>`,
+`SetSingleton<T>`, etc.) and generates a `ComponentRegistry` class with
+per-type serialize/deserialize callbacks via MessagePack.
+
+### 7.2 WorldSerializer
+
+```csharp
+static class WorldSerializer
+{
+    static void Save(World world, Stream stream);
+    static void Load(World world, Stream stream);
+}
+```
+
+Uses `ComponentRegistry` to serialize/deserialize all entities and their
+components to/from a MessagePack stream. `IRelationship.Source` is fixed
+up to the owning entity on load.
+
+### 7.3 Snapshot Types
+
+`WorldSnapshot`, `EntitySnapshot`, and `ComponentEntry` are the public
+serialization DTOs.
+
+---
+
+## Phase 8 — Polish & Documentation (Week 8)
+
+### 8.1 XML Docs
 
 All public API surface gets `<summary>` XML documentation comments.
 
-### 7.2 README
+### 8.2 README
 
 Quick-start guide with a minimal example: create world, register system, run
 tick, observe changes.
 
-### 7.3 NuGet Packaging
+### 8.3 NuGet Packaging
 
 `OECS.csproj` includes package metadata:
 - `PackageId`: `OECS`
 - `Description`: "Observable ECS for C# — an entity component system focused on
   a clean reactive API surface."
 - `PackageTags`: `ecs;reactive;observable;gamedev`
+- Bundles `OECS.SourceGen.dll` as an analyzer for consumers.
 
-### 7.4 CI (optional)
+### 8.4 Testing Games
 
-GitHub Actions workflow: build, test, pack.
+Test games (`Game.Blackjack`, `Game.TicTacToe`) serve as integration tests
+and design validation. See `docs/testing-games.md` for the testing strategy.
 
 ---
 
@@ -507,7 +572,8 @@ Phase 1 (Core)
         └─→ Phase 4 (Relationships)
               └─→ Phase 5 (Reactivity)
                     └─→ Phase 6 (Singletons)
-                          └─→ Phase 7 (Polish)
+                          └─→ Phase 7 (Source Gen & Serialization)
+                                └─→ Phase 8 (Polish)
 ```
 
 Phases 3 and 4 can be done in parallel; Phase 5 depends on both.
@@ -520,8 +586,8 @@ Phases 3 and 4 can be done in parallel; Phase 5 depends on both.
    default. If needed, systems could declare read/write component access for
    automatic parallel scheduling — but this adds significant complexity.
 
-2. **World serialization?** Since components are MessagePack-serializable,
+2. **Multiple worlds?** The design supports it naturally — `World` is a class,
-   snapshotting the entire world is feasible. This is a Phase 7+ stretch goal.
-
-3. **Multiple worlds?** The design supports it naturally — `World` is a class,
    you can instantiate multiple. No cross-world references are supported.
+
+3. **Component import from CSV/MasterMemory?** The `design.md` mentions this as
+   a potential feature. Not yet implemented.

docs/testing-games.md

@@ -0,0 +1,53 @@
+# Testing games
+
+1. Games are DLLs. Use a standalone test project to test a game.
+2. Create baseline game snapshots in textual format. Save to files.
+3. Create baseline logs via the R3 observable API. Save to files.
+4. Read the snapshot/logs manually to identify issues.
+5. Make sure serialization roundtrip works.
+
+## Play tests
+
+The goal of playtests is to execute a game with AI players and potentially
+random seeds. Then the log/snapshots are read by the LLM agent to spot issues.
+
+There is no failure in play tests; all tests pass but the agent should read the
+log for reference.
+
+AI players are not LLMs here; they are typically common game AI implementations.
+
+Create static functions that analyze the game state and return a command. Each
+such function is an agent. Each AI player routes its decisions to a random
+agent in a pool of weighted agents.
+
+Agents to consider:
+- **Random:** evenly chooses every move randomly.
+- **Greedy:** uses a specific way to score actions. Always chooses the one with
+  highest score. There can be multiple Greedy agents with different scoring.
+
+For each playtest attempt, generate a full game log, analyze and report what
+you find from the log.
+
+### Round Runner
+
+Playtests should run multiple rounds, not just one. The recommended pattern:
+
+```
+while chips >= minimum_bet AND chips < 2× starting_chips:
+    place bet → deal → agent decides hit/stand → resolve → new round
+```
+
+This exercises the full game lifecycle including deck reshuffling, round
+transitions, and chip management. Safety cap at ~200 rounds to prevent
+infinite loops.
+
+### Play Log Format
+
+Each play log should contain:
+- **Header:** agent name, seed, result summary, win/loss/push counts.
+- **Round Summaries:** per-round result, chip delta, hit count.
+- **Decisions:** per-decision hand total and choice (Hit/Stand).
+- **Reactivity:** raw R3 observable log of component/entity changes.
+- **Final State:** snapshot of the world after all rounds.
+
+Logs are saved as `.playlog` files alongside test output for manual review.

examples/Blackjack/Blackjack.csproj

@@ -1,20 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-
-  <PropertyGroup>
-    <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-    <RootNamespace>Blackjack</RootNamespace>
-  </PropertyGroup>
-
-  <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS\OECS.csproj" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <ProjectReference Include="..\..\src\OECS.SourceGen\OECS.SourceGen.csproj"
-                      OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
-  </ItemGroup>
-
-</Project>

examples/Blackjack/Program.cs

@@ -1,114 +0,0 @@
-using OECS;
-
-namespace Blackjack;
-
-public static class Program
-{
-    public static void Main()
-    {
-        var world = new World();
-
-        // ── Initialize game state ─────────────────────────────────────
-
-        // Use a seed based on current time, or a fixed seed for reproducibility.
-        uint seed = (uint)Environment.TickCount;
-        // Uncomment the next line for a deterministic game:
-        // seed = 12345u;
-
-        world.SetSingleton(new GameState
-        {
-            Phase = GamePhase.Betting,
-            Result = RoundResult.None,
-            Chips = 100,
-            CurrentBet = 0,
-            RoundNumber = 1,
-            Seed = seed
-        });
-        world.PostChanges();
-
-        // ── Wire up systems ───────────────────────────────────────────
-
-        var group = new SystemGroup(world);
-        group.Add(new DeckSetupSystem());
-        group.Add(new DealSystem());
-        group.Add(new PlayerBustCheckSystem());
-        group.Add(new DealerSystem());
-        group.Add(new RenderSystem());
-
-        // ── Game loop ─────────────────────────────────────────────────
-
-        group.RunLogical();
-
-        while (true)
-        {
-            var state = world.ReadSingleton<GameState>();
-
-            // Check for game over (out of chips).
-            if (state.Chips <= 0 && state.Phase == GamePhase.Betting)
-            {
-                Console.WriteLine();
-                Console.WriteLine("  You're out of chips! Game over.");
-                break;
-            }
-
-            switch (state.Phase)
-            {
-                case GamePhase.Betting:
-                    var input = Console.ReadLine();
-                    if (string.IsNullOrWhiteSpace(input))
-                        continue;
-                    if (input.Trim().ToLower() == "quit")
-                        return;
-
-                    if (int.TryParse(input.Trim(), out var bet))
-                    {
-                        world.Commands.Enqueue(new PlaceBetCommand { Amount = bet });
-                    }
-                    else
-                    {
-                        Console.WriteLine("  Invalid bet. Enter a number.");
-                        continue;
-                    }
-                    break;
-
-                case GamePhase.PlayerTurn:
-                    var choice = Console.ReadLine();
-                    if (string.IsNullOrWhiteSpace(choice))
-                        continue;
-                    if (choice.Trim().ToLower() == "quit")
-                        return;
-
-                    switch (choice.Trim().ToLower())
-                    {
-                        case "h":
-                        case "hit":
-                            world.Commands.Enqueue(new HitCommand());
-                            break;
-                        case "s":
-                        case "stand":
-                            world.Commands.Enqueue(new StandCommand());
-                            break;
-                        default:
-                            Console.WriteLine("  Invalid choice. Enter H or S.");
-                            continue;
-                    }
-                    break;
-
-                case GamePhase.RoundOver:
-                    Console.ReadLine();
-                    ref var roundState = ref world.GetSingleton<GameState>();
-                    roundState.RoundNumber++;
-                    world.MarkModified<GameState>(World.SingletonEntity);
-                    world.PostChanges();
-                    world.Commands.Enqueue(new NewRoundCommand());
-                    break;
-
-                default:
-                    // Dealing, DealerTurn — systems handle these automatically.
-                    break;
-            }
-
-            group.RunLogical();
-        }
-    }
-}

examples/Blackjack/Systems/RenderSystem.cs

@@ -1,148 +0,0 @@
-using OECS;
-
-namespace Blackjack;
-
-/// <summary>
-/// Renders the current game state to the console.
-/// </summary>
-public class RenderSystem : ISystem
-{
-    public void Run(World world)
-    {
-        var state = world.ReadSingleton<GameState>();
-
-        Console.WriteLine();
-        Console.WriteLine("  Blackjack");
-        Console.WriteLine("  ═════════");
-        Console.WriteLine();
-        Console.WriteLine($"  Chips: {state.Chips}  |  Round: {state.RoundNumber}");
-        Console.WriteLine();
-
-        // Show dealer's hand.
-        var dealerTotal = HandUtil.CalculateHand(world, DealerHandTag.Instance);
-        Console.Write("  Dealer: ");
-        if (state.Phase == GamePhase.PlayerTurn)
-        {
-            // Hide first card (hole card) during player's turn.
-            Console.Write("?? ");
-            PrintHand(world, DealerHandTag.Instance, skipFirst: true);
-        }
-        else
-        {
-            PrintHand(world, DealerHandTag.Instance);
-            Console.Write($"  ({dealerTotal})");
-        }
-        Console.WriteLine();
-
-        // Show player's hand.
-        var playerTotal = HandUtil.CalculateHand(world, PlayerHandTag.Instance);
-        Console.Write("  Player: ");
-        PrintHand(world, PlayerHandTag.Instance);
-        Console.Write($"  ({playerTotal})");
-        Console.WriteLine();
-        Console.WriteLine();
-
-        // Status line.
-        switch (state.Phase)
-        {
-            case GamePhase.Betting:
-                Console.Write($"  Enter bet (1–{state.Chips}): ");
-                break;
-            case GamePhase.PlayerTurn:
-                Console.Write("  [H]it or [S]tand: ");
-                break;
-            case GamePhase.RoundOver:
-                Console.WriteLine($"  {ResultText(state.Result)}");
-                Console.WriteLine();
-                Console.Write("  Press Enter for next round...");
-                break;
-            default:
-                break;
-        }
-    }
-
-    private static void PrintHand(World world, DealerHand hand, bool skipFirst = false)
-    {
-        PrintHandImpl(world, hand, skipFirst);
-    }
-
-    private static void PrintHand(World world, PlayerHand hand)
-    {
-        PrintHandImpl(world, hand, skipFirst: false);
-    }
-
-    private static void PrintHandImpl<T>(World world, T handTag, bool skipFirst)
-        where T : struct
-    {
-        Entity handEntity = Entity.Null;
-        using (var iter = world.Select<T>())
-        {
-            while (iter.MoveNext())
-            {
-                if (iter.CurrentEntity != World.SingletonEntity)
-                {
-                    handEntity = iter.CurrentEntity;
-                    break;
-                }
-            }
-        }
-
-        if (handEntity == Entity.Null)
-            return;
-
-        var cards = world.GetSources<Holds>(handEntity);
-        var ordered = cards.OrderByDescending(c =>
-        {
-            var card = world.ReadComponent<Card>(c);
-            return (int)card.Rank;
-        }).ToList();
-
-        bool first = true;
-        foreach (var cardEntity in ordered)
-        {
-            if (skipFirst && first)
-            {
-                first = false;
-                continue;
-            }
-            first = false;
-            var card = world.ReadComponent<Card>(cardEntity);
-            Console.Write(CardToString(card));
-            Console.Write(" ");
-        }
-    }
-
-    private static string CardToString(Card card)
-    {
-        var suit = card.Suit switch
-        {
-            Suit.Hearts => "♥",
-            Suit.Diamonds => "♦",
-            Suit.Clubs => "♣",
-            Suit.Spades => "♠",
-            _ => "?"
-        };
-
-        var rank = card.Rank switch
-        {
-            Rank.Ace => "A",
-            Rank.Jack => "J",
-            Rank.Queen => "Q",
-            Rank.King => "K",
-            _ => ((int)card.Rank).ToString()
-        };
-
-        return $"{rank}{suit}";
-    }
-
-    private static string ResultText(RoundResult result) => result switch
-    {
-        RoundResult.PlayerBust => "You bust! Dealer wins.",
-        RoundResult.DealerBust => "Dealer busts! You win!",
-        RoundResult.PlayerWin => "You win!",
-        RoundResult.DealerWin => "Dealer wins.",
-        RoundResult.Push => "Push — it's a tie!",
-        RoundResult.Blackjack => "Blackjack!",
-        _ => ""
-    };
-}

examples/TicTacToe/Program.cs

@@ -1,153 +0,0 @@
-using OECS;
-using R3;
-
-namespace TicTacToe;
-
-public static class Program
-{
-    private static readonly string SavePath = Path.Combine(
-        AppContext.BaseDirectory, "save.bin");
-
-    public static void Main()
-    {
-        var world = new World();
-        var reactivityLog = new List<string>();
-        SetupReactivityLogging(world, reactivityLog);
-
-        // ── Try to load a saved game ──────────────────────────────────
-
-        bool loaded = false;
-        if (File.Exists(SavePath))
-        {
-            Console.Write("Saved game found. Load it? (y/n): ");
-            if (Console.ReadLine()?.Trim().ToLower() == "y")
-            {
-                using var stream = File.OpenRead(SavePath);
-                WorldSerializer.Load(world, stream);
-                loaded = true;
-                Console.WriteLine("Game loaded!");
-                Console.WriteLine();
-            }
-        }
-
-        // ── Load fresh board from CSV (if not loaded) ─────────────────
-
-        if (!loaded)
-        {
-            var csvPath = Path.Combine(AppContext.BaseDirectory, "Data", "board.csv");
-            CsvLoader.LoadCells(world, csvPath);
-            world.PostChanges();
-
-            world.SetSingleton(new GameState
-            {
-                CurrentPlayer = Player.X,
-                Status = GameStatus.Playing,
-                MoveCount = 0
-            });
-            world.PostChanges();
-        }
-
-        // ── Wire up systems ───────────────────────────────────────────
-
-        var group = new SystemGroup(world);
-        group.Add(new WinCheckSystem());
-        group.Add(new RenderSystem());
-
-        // ── Game loop ─────────────────────────────────────────────────
-
-        group.RunLogical();
-        PrintReactivityLog(reactivityLog);
-
-        while (true)
-        {
-            ref var state = ref world.GetSingleton<GameState>();
-            if (state.Status != GameStatus.Playing)
-                break;
-
-            var input = Console.ReadLine();
-            if (string.IsNullOrWhiteSpace(input))
-                continue;
-
-            // "save" command.
-            if (input.Trim().ToLower() == "save")
-            {
-                SaveGame(world);
-                Console.WriteLine("  Game saved.");
-                continue;
-            }
-
-            var parts = input.Trim().Split(' ', StringSplitOptions.RemoveEmptyEntries);
-            if (parts.Length != 2 ||
-                !int.TryParse(parts[0], out var row) ||
-                !int.TryParse(parts[1], out var col))
-            {
-                Console.WriteLine("  Invalid input. Try \"1 2\" or \"save\".");
-                continue;
-            }
-
-            if (row < 0 || row > 2 || col < 0 || col > 2)
-            {
-                Console.WriteLine("  Row and col must be 0–2.");
-                continue;
-            }
-
-            world.Commands.Enqueue(new PlaceMarkCommand { Row = row, Col = col });
-            group.RunLogical();
-            PrintReactivityLog(reactivityLog);
-
-            // Auto-save after every move.
-            SaveGame(world);
-        }
-
-        // ── Final state ───────────────────────────────────────────────
-
-        Console.WriteLine();
-        Console.WriteLine("=== Game over ===");
-        PrintReactivityLog(reactivityLog);
-
-        // Clean up save file on game over.
-        if (File.Exists(SavePath))
-            File.Delete(SavePath);
-    }
-
-    private static void SaveGame(World world)
-    {
-        using var stream = File.Create(SavePath);
-        WorldSerializer.Save(world, stream);
-    }
-
-    private static void SetupReactivityLogging(World world, List<string> log)
-    {
-        world.ObserveEntityChanges().Subscribe(change =>
-        {
-            if (change.ComponentType != null) return;
-            log.Add($"[react] {change}");
-        });
-
-        world.ObserveComponentChanges<Mark>().Subscribe(change =>
-        {
-            log.Add($"[react] {change}");
-        });
-
-        world.ObserveComponentChanges<GameState>().Subscribe(change =>
-        {
-            log.Add($"[react] {change}");
-        });
-
-        var markedQuery = world.Query().With<Cell>().With<Mark>().Build();
-        world.ObserveQuery(markedQuery).Subscribe(change =>
-        {
-            log.Add($"[react:query] {change}");
-        });
-    }
-
-    private static void PrintReactivityLog(List<string> log)
-    {
-        if (log.Count == 0) return;
-        Console.WriteLine("── reactivity log ──");
-        foreach (var entry in log)
-            Console.WriteLine(entry);
-        Console.WriteLine();
-        log.Clear();
-    }
-}

examples/TicTacToe/Systems/RenderSystem.cs

@@ -1,61 +0,0 @@
-using OECS;
-
-namespace TicTacToe;
-
-/// <summary>
-/// Prints the board to the console.
-/// </summary>
-public class RenderSystem : ISystem
-{
-    public void Run(World world)
-    {
-        var state = world.ReadSingleton<GameState>();
-
-        // Build a 3×3 grid.
-        var grid = new char[3, 3];
-        for (int r = 0; r < 3; r++)
-            for (int c = 0; c < 3; c++)
-                grid[r, c] = '.';
-
-        // Fill in placed marks using the iterator API.
-        using var markIter = world.Select<Cell, Mark>();
-        while (markIter.MoveNext())
-        {
-            grid[markIter.Current1.Row, markIter.Current1.Col] =
-                markIter.Current2.Player == Player.X ? 'X' : 'O';
-        }
-
-        Console.WriteLine();
-        Console.WriteLine("  Tic-Tac-Toe");
-        Console.WriteLine("  ═══════════");
-        Console.WriteLine();
-        Console.WriteLine("    0   1   2");
-        Console.WriteLine("  ┌───┬───┬───┐");
-        for (int r = 0; r < 3; r++)
-        {
-            Console.Write($"{r} │");
-            for (int c = 0; c < 3; c++)
-            {
-                Console.Write($" {grid[r, c]} ");
-                if (c < 2) Console.Write("│");
-            }
-            Console.WriteLine("│");
-            if (r < 2) Console.WriteLine("  ├───┼───┼───┤");
-        }
-        Console.WriteLine("  └───┴───┴───┘");
-        Console.WriteLine();
-
-        // Status line.
-        var statusText = state.Status switch
-        {
-            GameStatus.Playing => $"{(state.CurrentPlayer == Player.X ? 'X' : 'O')}'s turn (move {state.MoveCount + 1})",
-            GameStatus.XWon => "X wins!",
-            GameStatus.OWon => "O wins!",
-            GameStatus.Draw => "It's a draw!",
-            _ => ""
-        };
-        Console.WriteLine($"  {statusText}");
-        Console.WriteLine();
-        Console.Write("  Enter row col (e.g. \"1 2\"): ");
-    }
-}

examples/TicTacToe/test_input.txt

@@ -1,5 +0,0 @@
-0 0
-1 1
-0 1
-1 0
-0 2

examples/TicTacToe/test_load.txt

@@ -1,3 +0,0 @@
-y
-1 0
-0 2

examples/TicTacToe/test_save.txt

@@ -1,3 +0,0 @@
-0 0
-1 1
-0 1

src/OECS/ComponentRegistry.cs

@@ -1,51 +0,0 @@
-using System.Runtime.CompilerServices;
-
-namespace OECS;
-
-/// <summary>
-/// Registry of component types discovered at compile time by the
-/// OECS.SourceGen incremental generator. The consuming project's
-/// generated code populates this via a module initializer, so no
-/// manual registration is needed.
-/// </summary>
-public static class ComponentRegistry
-{
-    private static ComponentDescriptor[]? _descriptors;
-    private static Dictionary<string, ComponentDescriptor>? _byTypeName;
-
-    /// <summary>
-    /// All discovered component descriptors. Populated automatically
-    /// by the source generator at module initialization.
-    /// </summary>
-    public static ComponentDescriptor[] Descriptors =>
-        _descriptors ?? Array.Empty<ComponentDescriptor>();
-
-    /// <summary>
-    /// Lookup by assembly-qualified type name. Populated automatically
-    /// by the source generator at module initialization.
-    /// </summary>
-    public static IReadOnlyDictionary<string, ComponentDescriptor> ByTypeName
-    {
-        get
-        {
-            if (_byTypeName == null)
-            {
-                var dict = new Dictionary<string, ComponentDescriptor>();
-                foreach (var desc in Descriptors)
-                    dict[desc.TypeName] = desc;
-                _byTypeName = dict;
-            }
-            return _byTypeName;
-        }
-    }
-
-    /// <summary>
-    /// Called by generated code to register discovered component types.
-    /// Must be called before any serialization occurs.
-    /// </summary>
-    public static void Initialize(ComponentDescriptor[] descriptors)
-    {
-        _descriptors = descriptors;
-        _byTypeName = null; // Rebuild on next access.
-    }
-}