feat: multi-lumières directionnelles + ponctuelles (Étape 12, Phase 4.2)

- Light (48 o) + MAX_LIGHTS=8 ; FrameUniforms étendu (ambient, lights[8], compteurs, _pad)
- resources/lights.rs : Lights (1 dir +Z par défaut, non-régression) + into_frame_array
- Scene : API déclarative add_directional_light / add_point_light / set_ambient / clear_lights
- Renderer::write_frame_uniforms upload les lumières/ambiant de la scène
- standard_shader.wgsl : struct Light + boucles d'accumulation (dir + ponctuelles, atténuation linéaire)
- exemple cube : 1 lumière ponctuelle chaude
- Docs : shaders/resources README, README roadmap, ROADMAP (item coché + Spot notée futur)
- Build/test/fmt OK ; non-régression par défaut (1 dir +Z + ambiant blanc)

README.md

@@ -186,3 +186,4 @@ The architecture docs live in `docs/tech/` and are written in **French**. Each d
 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.)
 9. ✅ **Window resize (Étape 11, Phase 4.4)** — `App::resize` reconfigures the surface (`Context::configure`) and recreates the depth texture (`Renderer::resize_depth`) together on each `WindowEvent::Resized`, so color and depth attachments always match. Guards against 0×0 (minimize). The surface format is re-synced to the Renderer and Scene if it ever changes. (Done 2026-09-18; verified at runtime on the `cube` example.)
+10. ✅ **Multi-lighting (Étape 12, Phase 4.2)** — the scene now carries a global light list (directional + point) with a white ambient, uploaded into the per-frame `FrameUniforms` array each frame. `Scene::add_directional_light` / `add_point_light` / `set_ambient` / `clear_lights` configure it; `FrameUniforms::default()` (one white directional along +Z + white ambient) reproduces the pre-multi-light look exactly. The `standard` fragment accumulates ambient + all lights; the `cube` example adds a warm point light on top of the default directional. (Done 2026-09-18.)