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/display_settings/src/DisplaySettingsTestInitializer.cs

@@ -27,7 +27,7 @@ namespace Mods.DisplaySettingsTest
 
             queued = true;
             GameLogger.Log("TEST", "DisplaySettingsTest: activating");
-            (Godot.Engine.GetMainLoop() as SceneTree)?.Root?.CallDeferred("add_child", new DisplaySettingsTestDriverNode(shots));
+            ModRoot.AddDeferred(new DisplaySettingsTestDriverNode(shots));
         }
     }
 }