debugMountInto method
Mounts this subtree into renderScene without a full Scene.
Scene construction touches the GPU context, which is unavailable
in unit tests, so this seam lets render-lifecycle tests exercise
the mount / unmount path (and the RenderItem registration it
drives) against a bare RenderScene.
Implementation
@visibleForTesting
void debugMountInto(RenderScene renderScene) => _mount(renderScene);