Root nodes join through ModRoot.AddDeferred, so a reload that unloads the mod first never hands Godot a freed node

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test_suites/new_game/src/NewGameTestInitializer.cs

@@ -19,7 +19,7 @@ namespace Mods.NewGameTest
 
             queued = true;
             GameLogger.Log("TEST", "NewGameTest: activating");
-            (Godot.Engine.GetMainLoop() as SceneTree)?.Root?.CallDeferred("add_child", new NewGameTestDriverNode());
+            ModRoot.AddDeferred(new NewGameTestDriverNode());
         }
     }
 }