refactor: reorganize project structure and move examples

- Move examples (Blackjack, TicTacToe) to the root directory
- Convert example projects from Console applications to Libraries
- Add Directory.Build.props for shared build settings
- Update solution file and project references to reflect new paths

Blackjack/Commands/HitCommand.cs

@@ -0,0 +1,58 @@
+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Player hits: draws one card from the deck to the player's hand.
+/// </summary>
+[MessagePackObject]
+public struct HitCommand : ICommand
+{
+    public void Execute(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Phase != GamePhase.PlayerTurn)
+            return;
+
+        DrawCard<PlayerHand>(world);
+    }
+
+    /// <summary>
+    /// Draws the top card from the deck and adds it to the given hand.
+    /// </summary>
+    internal static void DrawCard<THand>(World world)
+        where THand : struct
+    {
+        var singletonEntity = World.SingletonEntity;
+        var deckEntity = FindEntity<Deck>(world, singletonEntity);
+        var handEntity = FindEntity<THand>(world, singletonEntity);
+
+        if (deckEntity == Entity.Null || handEntity == Entity.Null)
+            return;
+
+        // Find a card still in the deck.
+        var cardsInDeck = world.GetSources<InDeck>(deckEntity);
+        if (cardsInDeck.Count == 0)
+            return;
+
+        var cardEntity = cardsInDeck.First();
+
+        // Remove from deck, add to hand.
+        world.RemoveComponent<InDeck>(cardEntity);
+        world.AddComponent(cardEntity, new Holds { Source = cardEntity, Target = handEntity });
+    }
+
+    internal static Entity FindEntity<T>(World world, Entity singletonEntity)
+        where T : struct
+    {
+        using var iter = world.Select<T>();
+        while (iter.MoveNext())
+        {
+            if (iter.CurrentEntity != singletonEntity)
+                return iter.CurrentEntity;
+        }
+        return Entity.Null;
+    }
+}

Blackjack/Commands/NewRoundCommand.cs

@@ -0,0 +1,53 @@
+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Starts a new round after the previous one ended.
+/// </summary>
+[MessagePackObject]
+public struct NewRoundCommand : ICommand
+{
+    public void Execute(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Phase != GamePhase.RoundOver)
+            return;
+
+        // Clear hands from previous round.
+        var singletonEntity = World.SingletonEntity;
+        var handEntities = new List<Entity>();
+        using (var iter = world.Select<PlayerHand>())
+        {
+            while (iter.MoveNext())
+            {
+                if (iter.CurrentEntity != singletonEntity)
+                    handEntities.Add(iter.CurrentEntity);
+            }
+        }
+        using (var iter = world.Select<DealerHand>())
+        {
+            while (iter.MoveNext())
+            {
+                if (iter.CurrentEntity != singletonEntity)
+                    handEntities.Add(iter.CurrentEntity);
+            }
+        }
+        foreach (var hand in handEntities)
+        {
+            var cards = world.GetSources<Holds>(hand);
+            var deckEntity = HitCommand.FindEntity<Deck>(world, singletonEntity);
+            foreach (var card in cards)
+            {
+                world.RemoveComponent<Holds>(card);
+                world.AddComponent(card, new InDeck { Source = card, Target = deckEntity });
+            }
+        }
+
+        state.Phase = GamePhase.Betting;
+        state.Result = RoundResult.None;
+        world.MarkModified<GameState>(World.SingletonEntity);
+    }
+}

Blackjack/Commands/PlaceBetCommand.cs

@@ -0,0 +1,29 @@
+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Places a bet and advances the game to the dealing phase.
+/// </summary>
+[MessagePackObject]
+public struct PlaceBetCommand : ICommand
+{
+    [Key(0)] public int Amount;
+
+    public void Execute(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Phase != GamePhase.Betting)
+            return;
+
+        if (Amount < 1 || Amount > state.Chips)
+            return;
+
+        state.CurrentBet = Amount;
+        state.Chips -= Amount;
+        state.Phase = GamePhase.Dealing;
+        world.MarkModified<GameState>(World.SingletonEntity);
+    }
+}

Blackjack/Commands/StandCommand.cs

@@ -0,0 +1,22 @@
+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Player stands: advance to the dealer's turn.
+/// </summary>
+[MessagePackObject]
+public struct StandCommand : ICommand
+{
+    public void Execute(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Phase != GamePhase.PlayerTurn)
+            return;
+
+        state.Phase = GamePhase.DealerTurn;
+        world.MarkModified<GameState>(World.SingletonEntity);
+    }
+}