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/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;
-        using (var iter = world.Select<T>())
-        {
-            while (iter.MoveNext())
-            {
-                if (iter.CurrentEntity != World.SingletonEntity)
-                {
-                    handEntity = iter.CurrentEntity;
-                    break;
-                }
-            }
-        }
+        var handEntity = world.FindEntity<T>();
 
         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);
     }
 }