- Replace the broken App::new() (which does not exist) with AppBuilder.
- Strip it down to the PLAN's ~15-line target: no explicit winit/wgpu,
AppHandler render() left empty (scene rendering still on the roadmap).
- Verified: builds, opens a black window and runs the update->render->present
loop until closed.
- Update README/PLAN wording now that the simple example compiles (it still
does not draw a scene).