docs: mark Étape 7 done (Scene cache, Mesh->Material), update PLAN/ROADMAP/README

DRAFT Étape 7: all 24 boxes checked + point d'étape 2026-09-17. PLAN Phase 2
three lines checked (cache in Scene, Mesh->Material, standard_shader default).
ROADMAP 1.2 material facet noted done while the Arc<Geometry> refactor stays
deferred. README declarative snippet updated to register_shader /
add_material_shader / create_mesh / add_entity.

README.md

@@ -93,19 +93,22 @@ impl AppHandler for MyGame {
 async fn main() -> Result<(), wsg_lib::utils::WsgError> {
     let app = AppBuilder::new().build().await?;
 
-    // Register your scene once (string IDs), then App renders it automatically each frame:
+    // Register your scene once (string IDs), then App renders it automatically each frame.
+    // Since Étape 7 the Scene owns the pipeline cache: build materials/meshes through it and
+    // link the material to the mesh (no material_id on the entity anymore).
     //   app.renderer_mut().set_unlit(true); // select flat 2D rendering (optional)
-    //   app.cache.register_shader("standard", wsg_lib::utils::STANDARD_SHADER_PATH)?;
-    //   app.scene.add_mesh("quad", Arc::new(mesh))?;
-    //   app.scene.add_material("mat", Arc::new(Material::new(app.renderer.format(), "standard", &mut app.cache)))?;
-    //   app.scene.add_entity("my_quad", "quad", "mat")?;
+    //   app.scene.register_shader("standard", wsg_lib::utils::STANDARD_SHADER_PATH)?;
+    //   app.scene.add_material_shader("mat", "standard")?;                    // build via the Scene's cache
+    //   app.scene.create_mesh("quad", &vertices, Some(&indices), Some("mat"))?; // mesh links its Material
+    //   app.scene.add_entity("my_quad", "quad")?;
 
     app.run(MyGame)
 }
 ```
 
-> API note: `Scene::add_mesh` / `add_material` / `add_entity` and `PipelineCache::register_shader`
-> currently return `Result<_, String>` — typed error unification is on the roadmap.
+> API note: `Scene::register_shader` / `add_material_shader` / `create_mesh` / `add_entity` and
+> `PipelineCache::register_shader` currently return `Result<_, String>` — typed error unification
+> is on the roadmap.
 
 ## Architecture overview