refactor(resources): Scene owns pipeline cache, Mesh->Material link

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.

lib/src/core/renderer.rs

@@ -248,12 +248,18 @@ impl Renderer {
                 ..Default::default()
             });
 
-            for (label, mesh, material, transform) in scene.iter_entities() {
+            // Étape 7 (DRAFT 7.3.4) : the Material is resolved from the Mesh itself, falling back
+            // to the Scene's default material when the mesh carries none.
+            for (label, mesh, transform) in scene.iter_entities() {
+                let material = mesh
+                    .material()
+                    .cloned()
+                    .unwrap_or_else(|| scene.default_material());
                 let object_bind_group = self.object_bind_group_for(label, transform);
                 draw_entity(
                     &mut render_pass,
                     mesh,
-                    material,
+                    &material,
                     &self.frame_bind_group,
                     &object_bind_group,
                 );