fix: shadow caster index, torus winding, per-frame device poll ; close stage 15

Bug fixes found while validating stage 15:
- demo: set_shadow_caster(Some(0)) selected the default +Z light (packed
  index 0 from Lights::new()); the demo's warm directional light is packed
  at index 1. The shadow camera then looked down -Z, so misaligned objects
  occluded each other (cone/torus rendered black). Use index 1.
- primitives::torus: index winding was flipped ([a,c,b]); the outer surface
  (outward normals, CCW from outside) was culled and only the dark interior
  stayed visible. Reversed to [a,b,c]/[b,d,c] so it is CCW from outside.
- app: call device.poll() each frame in about_to_wait; without it wgpu
  async callbacks (on_submitted_work_done, map_async) never fire in the
  windowed loop.

Docs:
- conf: clarify the embedded-shader fallback is expected/harmless and that
  SHADOW_SHADER_PATH is kept for API compatibility only.
- README item 15: runtime-verified headless.
- DRAFT.md: emptied to a completion summary per convention (full stage-15
  plan preserved in git history).

README.md

@@ -191,4 +191,4 @@ The architecture docs live in `docs/tech/` and are written in **French**. Each d
 12. ✅ **Shadows — shadow mapping (Étape 14, Phase 4.2, optionnel)** — classic two-pass shadow mapping on a **single** light (directional or spot), selected by `Scene::set_shadow_caster(index)`. A depth-only pass (`shadow_shader.wgsl` + dedicated `shadow_pipeline`) renders the scene into a 1024² `Depth32Float` shadow map (`Renderer`-owned, slope-scaled depth bias); the `standard` fragment re-projects each fragment into light space and applies a **PCF 3×3** comparison-sampler test (bind group **@3**, shared). `FrameUniforms` grew from 704 to 784 bytes (`shadow_light_index`, `light_view_proj`, `shadow_params`). Shadows are **off by default** (`shadow_caster = None`) so `simple`/`cube`/`manual`/`spot_test` are unchanged. The `shadow_test` example casts a soft shadow from a cube onto a ground slab. (Done 2026-09-19.)
 13. ✅ **Procedural primitive meshes (Étape 15.A)** — `math::primitives` provides drop-in `Geometry` generators (`cube`, `plane`, `uv_sphere`, `icosphere`, `cylinder`, `cone`, `torus`) with positions + per-face/smooth normals + UVs + indices. Re-exported at `math::*`. The `cube` and `spot_test` examples now reuse `math::cube(1.0)` (the `cube_geometry` helper was factored away; `shadow_test` keeps its generic `box_geometry`). (Done 2026-09-20; 6 unit tests.)
 14. ✅ **Unified input (Étape 15.B)** — `core::input::InputState` gives cross-frame **pressed/held/released** semantics for keyboard (physical `KeyCode`) and mouse (buttons, position, per-frame delta, wheel scroll), rotated by `begin_frame`/`end_frame` around `AppHandler::update`. `App` exposes it as a public `input` field, fed from winit `WindowEvent`s and reset each frame. Gamepad is reserved/deferred (DRAFT D7). (Done 2026-09-20; 5 unit tests; winit event handling is host-driven on the CPU, not WGSL.)
-15. ✅ **Orbital camera + final demo (Étape 15.C)** — `resources::CameraController` (yaw/pitch/distance/target, `apply_to` writes into a `Camera`, drag-orbit + wheel-zoom + clamps) drives the new `demo` example: one of each primitive, procedural textures, standard Phong material, a shadow-casting directional light + point + spot, and live mouse-orbit / wheel-zoom / `R` reset / `1`/`2`/`3` view presets. Run with `cargo run -p wsg-lib --example demo`. (Done 2026-09-20; runtime check pending.)
+15. ✅ **Orbital camera + final demo (Étape 15.C)** — `resources::CameraController` (yaw/pitch/distance/target, `apply_to` writes into a `Camera`, drag-orbit + wheel-zoom + clamps) drives the new `demo` example: one of each primitive, procedural textures, standard Phong material, a shadow-casting directional light + point + spot, and live mouse-orbit / wheel-zoom / `R` reset / `1`/`2`/`3` view presets. Run with `cargo run -p wsg-lib --example demo`. (Done 2026-09-20; runtime-verified headless.)