refactor: use ReadSingleton for GameState in RenderSystem

examples/TicTacToe/Systems/RenderSystem.cs

@@ -16,7 +16,7 @@ public class RenderSystem : ISystem
 
     public void Run(World world)
     {
-        ref var state = ref world.GetSingleton<GameState>();
+        var state = world.ReadSingleton<GameState>();
 
         // Build a 3×3 grid.
         var grid = new char[3, 3];