refactor: update iterator access and simplify loops

Refactor code to use `Val1` and `Val2` instead of `Item1` and `Item2`
for
iterator access. Simplify `PlaceMarkCommand` and `WinCheckSystem` by
replacing `while` loops with `foreach` and removing redundant comments.

Game.TicTacToe/Systems/WinCheckSystem.cs

@@ -8,24 +8,18 @@ namespace Game.TicTacToe;
 /// </summary>
 public class WinCheckSystem : ISystem
 {
-    public void Run(World world)
+    public void RunImpl(World world)
     {
-        // Check game status before doing any work. Use ReadSingleton
-        // to avoid auto-marking GameState as modified when we bail early.
         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.
         var grid = new Player[3, 3];
-
-        while (iter.MoveNext())
+        foreach (var iter in world.Select<Cell, Mark>())
         {
-            grid[iter.Item1.Row, iter.Item1.Col] = iter.Item2.Player;
+            grid[iter.Val1.Row, iter.Val1.Col] = iter.Val2.Player;
         }
 
         // Check rows.
@@ -62,9 +56,7 @@ public class WinCheckSystem : ISystem
 
         // Check draw.
         if (state.MoveCount >= 9)
-        {
             state.Status = GameStatus.Draw;
-        }
     }
 
     private static bool TryGetWinner(Player a, Player b, Player c, out Player winner)
@@ -82,4 +74,4 @@ public class WinCheckSystem : ISystem
     {
         state.Status = winner == Player.X ? GameStatus.XWon : GameStatus.OWon;
     }
-}
+}