refactor: simplify component iteration and singleton handling

Update the ECS engine to support automatic mutation tracking for
singletons during iteration, removing the need for manual
`MarkModified` calls.

- Update `ComponentDiscoveryGenerator` to detect singleton components.
- Refactor `PlaceMarkCommand` and `WinCheckSystem` to leverage
  automatic mutation marking.
- Replace `ForEach` usage with `Select` and `ItemN` access in tests
  and systems to align with the updated iterator API.

Game.TicTacToe/Commands/PlaceMarkCommand.cs

@@ -15,22 +15,23 @@ public struct PlaceMarkCommand : ICommand
 
     public void Execute(World world)
     {
-        ref var state = ref world.GetSingleton<GameState>();
-
-        if (state.Status != GameStatus.Playing)
+        // Read-only check before iteration — never auto-marks.
+        if (world.ReadSingleton<GameState>().Status != GameStatus.Playing)
             return;
 
         // Find the cell entity at (Row, Col) that has no Mark.
-        // Use the iterator API with early-exit via break.
-        var query = world.Query().With<Cell>().Without<Mark>().Build();
+        var query = new Query<Cell>().Without<Mark>();
         Entity? target = null;
 
-        using var iter = world.Select<Cell>(query);
+        // Fetch singleton ref inside iteration so mutations are auto-marked.
+        using var iter = world.Select(query);
+        ref var state = ref world.GetSingleton<GameState>();
+
         while (iter.MoveNext())
         {
-            if (iter.Current1.Row == Row && iter.Current1.Col == Col)
+            if (iter.Item1.Row == Row && iter.Item1.Col == Col)
             {
-                target = iter.CurrentEntity;
+                target = iter.Entity;
                 break;
             }
         }
@@ -38,13 +39,11 @@ public struct PlaceMarkCommand : ICommand
         if (target == null)
             return; // Cell already occupied or invalid position.
 
-        // Place the mark. ComponentAdded is sufficient — MarkModified
-        // is only needed when mutating an existing component via GetComponent<T>.
+        // Place the mark.
         world.AddComponent(target.Value, new Mark { Player = state.CurrentPlayer });
 
-        // Advance turn.
+        // Advance turn. Mutations auto-marked via EndIteration — no MarkModified needed.
         state.MoveCount++;
         state.CurrentPlayer = state.CurrentPlayer == Player.X ? Player.O : Player.X;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }