docs(resources): Étape 10 textures — bilan, README et ROADMAP à jour

- ROADMAP : Phase 4.1 (Textures) cochée — struct Texture, uvs, bind group
  shader, Material avec texture diffuse.
- README : jalon 8 (diffuse textures) ajouté à la Roadmap ; ligne resource
  'texture' + material enrichi dans le tableau des resources du module.
- DRAFT Étape 10 : cases 10.1-10.6 cochées (terminé et vérifié le
  2026-09-18), point d'étape clôturé, bilan de fin d'étape rédigé (y compris
  la divergence D2 : multiplication texel * couleur du vertex au lieu de
  remplacement, et la structure réelle du PipelineCache).

README.md

@@ -184,3 +184,4 @@ The architecture docs live in `docs/tech/` and are written in **French**. Each d
 5. **Typed resource handles** — keep String IDs for the MVP (current design, source of truth in `Scene`); slotmap-based generational handles (`ARCHI_ARENES.md`) are deferred to a later performance pass.
 6. **Error unification** — replace `Result<_, String>` in `Scene`/`PipelineCache` with typed errors.
 7. ✅ **CPU geometry storage (Étape 8)** — `Mesh` retains a shared `Arc<Geometry>` (CPU source of truth with colors) alongside its GPU buffers; meshes are declared from a `Geometry` via `Mesh::from_geometry`/`Scene::create_mesh(id, geometry, material)` instead of raw `&[Vertex]` arrays. (Done 2026-09-18; `transform` stays on `Entity` — deviation D3.)
+8. ✅ **Diffuse textures (Étape 10, Phase 4.1)** — `resources::Texture` (GPU image: device+view+sampler, `Rgba8UnormSrgb`, loaders `from_rgba8`/`from_bytes`/`from_file`) attached to a `Material` as diffuse texture. The `standard` shader samples it via bind group **@2** (shared layout: sampler+texture); UVs are forwarded as vertex attribute location 2. Without a texture the material uses a shared 1×1 white placeholder so lit and unlit rendering are unchanged (no regression). The `cube` example now uses a procedural checkerboard texture. (Done 2026-09-18.)