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/Systems/WinCheckSystem.cs

@@ -15,15 +15,17 @@ public class WinCheckSystem : ISystem
         if (world.ReadSingleton<GameState>().Status != GameStatus.Playing)
             return;
 
+        // Get singleton ref inside iteration so all mutations are auto-marked
+        // by EndIteration — no MarkModified calls needed.
+        using var iter = world.Select<Cell, Mark>();
         ref var state = ref world.GetSingleton<GameState>();
 
-        // Build a 3×3 grid of marks using the iterator API.
+        // Build a 3×3 grid of marks.
         var grid = new Player[3, 3];
 
-        using var iter = world.Select<Cell, Mark>();
         while (iter.MoveNext())
         {
-            grid[iter.Current1.Row, iter.Current1.Col] = iter.Current2.Player;
+            grid[iter.Item1.Row, iter.Item1.Col] = iter.Item2.Player;
         }
 
         // Check rows.
@@ -31,7 +33,7 @@ public class WinCheckSystem : ISystem
         {
             if (TryGetWinner(grid[r, 0], grid[r, 1], grid[r, 2], out var winner))
             {
-                SetWinner(world, ref state, winner);
+                SetWinner(ref state, winner);
                 return;
             }
         }
@@ -41,7 +43,7 @@ public class WinCheckSystem : ISystem
         {
             if (TryGetWinner(grid[0, c], grid[1, c], grid[2, c], out var winner))
             {
-                SetWinner(world, ref state, winner);
+                SetWinner(ref state, winner);
                 return;
             }
         }
@@ -49,12 +51,12 @@ public class WinCheckSystem : ISystem
         // Check diagonals.
         if (TryGetWinner(grid[0, 0], grid[1, 1], grid[2, 2], out var diag1))
         {
-            SetWinner(world, ref state, diag1);
+            SetWinner(ref state, diag1);
             return;
         }
         if (TryGetWinner(grid[0, 2], grid[1, 1], grid[2, 0], out var diag2))
         {
-            SetWinner(world, ref state, diag2);
+            SetWinner(ref state, diag2);
             return;
         }
 
@@ -62,7 +64,6 @@ public class WinCheckSystem : ISystem
         if (state.MoveCount >= 9)
         {
             state.Status = GameStatus.Draw;
-            world.MarkModified<GameState>(World.SingletonEntity);
         }
     }
 
@@ -77,9 +78,8 @@ public class WinCheckSystem : ISystem
         return false;
     }
 
-    private static void SetWinner(World world, ref GameState state, Player winner)
+    private static void SetWinner(ref GameState state, Player winner)
     {
         state.Status = winner == Player.X ? GameStatus.XWon : GameStatus.OWon;
-        world.MarkModified<GameState>(World.SingletonEntity);
     }
 }