Étape 7 (DRAFT): the PipelineCache moves into the Scene (SceneGpu holds
device + format + cache, wired via Scene::init_gpu in AppRunner::resumed),
so App no longer owns a cache field. Mesh now holds Option<Arc<Material>>
(with material()/set_material()/with_material()); Entity drops material_id
({mesh_id, transform}); iter_entities yields (label, &Mesh, &Transform);
Renderer::render_scene resolves the material from the mesh or the Scene's
lazy default_material(). New declarative Scene helpers: register_shader,
add_material_shader, create_mesh. cube/simple examples migrated; manual
remains low-level and unchanged.
Jérôme Bousquié