feat: implement game saving and loading in TicTacToe

- Add serialization support to `World`, `EntityAllocator`, and
  `SparseSet`
- Implement `SaveGame` and `LoadGame` functionality in the TicTacToe
  example
- Add ability to resume a game from a saved state
- Implement auto-save after every move in the game loop

examples/TicTacToe/Commands/PlaceMarkCommand.cs

@@ -38,9 +38,9 @@ public struct PlaceMarkCommand : ICommand
         if (target == null)
             return; // Cell already occupied or invalid position.
 
-        // Place the mark.
+        // Place the mark. ComponentAdded is sufficient — MarkModified
+        // is only needed when mutating an existing component via GetComponent<T>.
         world.AddComponent(target.Value, new Mark { Player = state.CurrentPlayer });
-        world.MarkModified<Mark>(target.Value);
 
         // Advance turn.
         state.MoveCount++;