feat: lumières spot (cône + angle) (Étape 13, Phase 4.2)

- Light étendu à 4×Vec4 (64 o) : dir_angle (axe du cône + cos demi-angle)
- FrameUniforms 576→704 o : compteur num_spot, _pad[1]
- Lights + spot: Vec<Light> ; into_frame_array renvoie (arr, n_dir, n_point, n_spot)
- Scene::add_spot_light(pos, dir, color, intensity, radius, half_angle) ; clear_lights inclut spot
- standard_shader.wgsl : 3e boucle d'accumulation (pénombre lissée ±0.1 rad + atténuation linéaire)
- exemple cube : lumière spot verte pointée vers le cube
- Docs : shaders/resources README (704 B), README roadmap (item 11), ROADMAP (item coché)
- Build/test/fmt OK (5 tests + validation WGSL + doctests) ; non-régression par défaut

README.md

@@ -187,3 +187,4 @@ The architecture docs live in `docs/tech/` and are written in **French**. Each d
 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.)
+11. ✅ **Spot lights (Étape 13, Phase 4.2)** — spot lights (oriented cone + half-angle) added on top of the multi-lighting system. `Scene::add_spot_light(pos, dir, color, intensity, radius, half_angle)` registers a spot light; the `standard` fragment accumulates a spot term with a smoothed penumbra (half-angle ± 0.1 rad) and linear attenuation. `Light` grew from 48 to 64 bytes (added `dir_angle`); `FrameUniforms` from 576 to 704 bytes (added `num_spot`). Non-regression: default scene unchanged. The `cube` example adds a green spot light aimed at the cube. (Done 2026-09-18.)