refactor: simplify entity lookups and clean up state updates

- Replace manual entity iteration with `world.FindEntity<T>()`
- Remove redundant `MarkModified<GameState>` calls
- Add safety counters to prevent infinite loops in tests and systems
- Call `FlushPendingMutations` after card transfers to ensure
  consistency
- Rename `Run` to `RunImpl` in several systems

Game.Blackjack.Tests/GameFlowTests.cs

@@ -232,13 +232,7 @@ public class GameFlowTests
 
     private static Entity FindEntity<T>(World world) where T : struct
     {
-        using var iter = world.Select<T>();
+        return world.FindEntity<T>();
-        while (iter.MoveNext())
-        {
-            if (iter.CurrentEntity != World.SingletonEntity)
-                return iter.CurrentEntity;
-        }
-        return Entity.Null;
     }
 
     private static int CountCardsInHand(World world, Entity handEntity)
@@ -251,11 +245,14 @@ public class GameFlowTests
     private static List<(Suit, Rank)> GetAllCards(World world)
     {
         var cards = new List<(Suit, Rank)>();
-        var query = world.Query().With<Card>().Build();
+        using (var iter = world.Select<Card>())
-        world.ForEach(query, (Entity e, ref Card card) =>
         {
-            cards.Add((card.Suit, card.Rank));
+            while (iter.MoveNext())
-        });
+            {
+                var card = iter.Val1;
+                cards.Add((card.Suit, card.Rank));
+            }
+        }
         cards.Sort((a, b) =>
         {
             int cmp = a.Item1.CompareTo(b.Item1);

Game.Blackjack.Tests/PlayTests.cs

@@ -199,8 +199,10 @@ public class PlayTests
 
             // Player turn: agent decides hit/stand.
             int roundHits = 0;
-            while (world.ReadSingleton<GameState>().Phase == GamePhase.PlayerTurn)
+            int playerTurnSafety = 0;
+            while (world.ReadSingleton<GameState>().Phase == GamePhase.PlayerTurn && playerTurnSafety < 52)
             {
+                playerTurnSafety++;
                 var total = HandUtil.CalculateHand(world, new PlayerHand());
                 var decision = agent.Decide(world);
                 log.Decisions.Add($"R{totalRounds + 1} Hand={total}, Decision={decision}");
@@ -282,11 +284,7 @@ public class PlayTests
 
     private static Entity FindEntity<T>(World world) where T : struct
     {
-        using var iter = world.Select<T>();
+        return world.FindEntity<T>();
-        while (iter.MoveNext())
-            if (iter.CurrentEntity != World.SingletonEntity)
-                return iter.CurrentEntity;
-        return Entity.Null;
     }
 
     private static List<string> GetHandCards(World world, Entity hand)

Game.Blackjack.Tests/SnapshotTests.cs

@@ -248,12 +248,6 @@ public class SnapshotTests
 
     private static Entity FindEntity<T>(World world) where T : struct
     {
-        using var iter = world.Select<T>();
+        return world.FindEntity<T>();
-        while (iter.MoveNext())
-        {
-            if (iter.CurrentEntity != World.SingletonEntity)
-                return iter.CurrentEntity;
-        }
-        return Entity.Null;
     }
 }

Game.Blackjack/Commands/HitCommand.cs

@@ -11,7 +11,7 @@ public struct HitCommand : ICommand
 {
     public void Execute(World world)
     {
-        ref var state = ref world.GetSingleton<GameState>();
+        var state = world.ReadSingleton<GameState>();
 
         if (state.Phase != GamePhase.PlayerTurn)
             return;
@@ -25,9 +25,8 @@ public struct HitCommand : ICommand
     internal static void DrawCard<THand>(World world)
         where THand : struct
     {
-        var singletonEntity = World.SingletonEntity;
+        var deckEntity = world.FindEntity<Deck>();
-        var deckEntity = FindEntity<Deck>(world, singletonEntity);
+        var handEntity = world.FindEntity<THand>();
-        var handEntity = FindEntity<THand>(world, singletonEntity);
 
         if (deckEntity == Entity.Null || handEntity == Entity.Null)
             return;
@@ -42,17 +41,14 @@ public struct HitCommand : ICommand
         // Remove from deck, add to hand.
         world.RemoveComponent<InDeck>(cardEntity);
         world.AddComponent(cardEntity, new Holds { Target = handEntity });
+
+        // Flush so subsequent DrawCard calls see the updated state.
+        world.FlushPendingMutations();
     }
 
-    internal static Entity FindEntity<T>(World world, Entity singletonEntity)
+    internal static Entity FindEntity<T>(World world)
         where T : struct
     {
-        using var iter = world.Select<T>();
+        return world.FindEntity<T>();
-        while (iter.MoveNext())
-        {
-            if (iter.CurrentEntity != singletonEntity)
-                return iter.CurrentEntity;
-        }
-        return Entity.Null;
     }
-}
+}

Game.Blackjack/Commands/NewRoundCommand.cs

@@ -17,28 +17,25 @@ public struct NewRoundCommand : ICommand
             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.Entity);
-                    handEntities.Add(iter.CurrentEntity);
             }
         }
         using (var iter = world.Select<DealerHand>())
         {
             while (iter.MoveNext())
             {
-                if (iter.CurrentEntity != singletonEntity)
+                handEntities.Add(iter.Entity);
-                    handEntities.Add(iter.CurrentEntity);
             }
         }
         foreach (var hand in handEntities)
         {
             var cards = world.GetSources<Holds>(hand);
-            var deckEntity = HitCommand.FindEntity<Deck>(world, singletonEntity);
+            var deckEntity = world.FindEntity<Deck>();
             foreach (var card in cards)
             {
                 world.RemoveComponent<Holds>(card);
@@ -49,6 +46,5 @@ public struct NewRoundCommand : ICommand
         state.RoundNumber++;
         state.Phase = GamePhase.Betting;
         state.Result = RoundResult.None;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }

Game.Blackjack/Commands/PlaceBetCommand.cs

@@ -24,6 +24,5 @@ public struct PlaceBetCommand : ICommand
         state.CurrentBet = Amount;
         state.Chips -= Amount;
         state.Phase = GamePhase.Dealing;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }

Game.Blackjack/Commands/StandCommand.cs

@@ -17,6 +17,5 @@ public struct StandCommand : ICommand
             return;
 
         state.Phase = GamePhase.DealerTurn;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }

Game.Blackjack/Systems/DealSystem.cs

@@ -8,7 +8,7 @@ namespace Game.Blackjack;
 /// </summary>
 public class DealSystem : ISystem
 {
-    public void Run(World world)
+    public void RunImpl(World world)
     {
         var state = world.ReadSingleton<GameState>();
         if (state.Phase != GamePhase.Dealing)
@@ -23,7 +23,6 @@ public class DealSystem : ISystem
         HitCommand.DrawCard<DealerHand>(world);
 
         mutableState.Phase = GamePhase.PlayerTurn;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }
 

Game.Blackjack/Systems/DealerSystem.cs

@@ -8,7 +8,7 @@ namespace Game.Blackjack;
 /// </summary>
 public class DealerSystem : ISystem
 {
-    public void Run(World world)
+    public void RunImpl(World world)
     {
         var state = world.ReadSingleton<GameState>();
         if (state.Phase != GamePhase.DealerTurn)
@@ -18,10 +18,12 @@ public class DealerSystem : ISystem
 
         // Dealer must hit on 16 and below, stand on 17+.
         int dealerTotal = HandUtil.CalculateHand(world, DealerHandTag.Instance);
-        while (dealerTotal < 17)
+        int safety = 0;
+        while (dealerTotal < 17 && safety < 52)
         {
             HitCommand.DrawCard<DealerHand>(world);
             dealerTotal = HandUtil.CalculateHand(world, DealerHandTag.Instance);
+            safety++;
         }
 
         int playerTotal = HandUtil.CalculateHand(world, PlayerHandTag.Instance);
@@ -49,6 +51,5 @@ public class DealerSystem : ISystem
             mutableState.Chips += mutableState.CurrentBet;
         }
 
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }

Game.Blackjack/Systems/DeckSetupSystem.cs

@@ -9,18 +9,17 @@ namespace Game.Blackjack;
 /// </summary>
 public class DeckSetupSystem : ISystem
 {
-    public void Run(World world)
+    public void RunImpl(World world)
     {
         var state = world.ReadSingleton<GameState>();
         if (state.Phase != GamePhase.Dealing)
             return;
 
         // Only create deck entities if they don't exist yet.
-        var singletonEntity = World.SingletonEntity;
         bool hasDeck = false;
         using (var iter = world.Select<Deck>())
         {
-            hasDeck = iter.MoveNext() && iter.CurrentEntity != singletonEntity;
+            hasDeck = iter.MoveNext();
         }
 
         if (!hasDeck)
@@ -50,7 +49,7 @@ public class DeckSetupSystem : ISystem
 
         // Shuffle the deck using mulberry32 with the current seed.
         ref var mutableState = ref world.GetSingleton<GameState>();
-        var deckEntity2 = HitCommand.FindEntity<Deck>(world, singletonEntity);
+        var deckEntity2 = world.FindEntity<Deck>();
         var cards = world.GetSources<InDeck>(deckEntity2).ToArray();
         Shuffle(world, deckEntity2, cards, ref mutableState.Seed);
     }

Game.Blackjack/Systems/HandUtil.cs

@@ -22,18 +22,7 @@ public static class HandUtil
         where T : struct
     {
         // Find the hand entity.
-        Entity handEntity = Entity.Null;
+        var handEntity = world.FindEntity<T>();
-        using (var iter = world.Select<T>())
-        {
-            while (iter.MoveNext())
-            {
-                if (iter.CurrentEntity != World.SingletonEntity)
-                {
-                    handEntity = iter.CurrentEntity;
-                    break;
-                }
-            }
-        }
 
         if (handEntity == Entity.Null)
             return 0;

Game.Blackjack/Systems/PlayerBustCheckSystem.cs

@@ -8,7 +8,7 @@ namespace Game.Blackjack;
 /// </summary>
 public class PlayerBustCheckSystem : ISystem
 {
-    public void Run(World world)
+    public void RunImpl(World world)
     {
         var state = world.ReadSingleton<GameState>();
         if (state.Phase != GamePhase.PlayerTurn)
@@ -21,6 +21,5 @@ public class PlayerBustCheckSystem : ISystem
         ref var mutableState = ref world.GetSingleton<GameState>();
         mutableState.Phase = GamePhase.RoundOver;
         mutableState.Result = RoundResult.PlayerBust;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }