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/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 = FindEntity<Deck>(world, singletonEntity);
-        var handEntity = FindEntity<THand>(world, singletonEntity);
+        var deckEntity = world.FindEntity<Deck>();
+        var handEntity = world.FindEntity<THand>();
 
         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>();
-        while (iter.MoveNext())
-        {
-            if (iter.CurrentEntity != singletonEntity)
-                return iter.CurrentEntity;
-        }
-        return Entity.Null;
+        return world.FindEntity<T>();
     }
-}
+}

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.CurrentEntity);
+                handEntities.Add(iter.Entity);
             }
         }
         using (var iter = world.Select<DealerHand>())
         {
             while (iter.MoveNext())
             {
-                if (iter.CurrentEntity != singletonEntity)
-                    handEntities.Add(iter.CurrentEntity);
+                handEntities.Add(iter.Entity);
             }
         }
         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);
     }
 }