feat(particles): Étape 28 A — ParticlePool infrastructure (no driver)

Creates the GPU resource layer for particles per ARCHI §3.2/§6, without
simulation: the pool owns all buffers/pipeline/bind group but draws nothing
(indirect args zeroed → no-op) until a driver is attached (Étape B).

New:
- resources/particle.rs: Particle (80 B, #[repr(C)], no padding — D19),
  SIZE/ZERO consts + offset/layout unit tests
- shaders/particle_billboard.wgsl: camera-facing billboard, empty vertex
  layout (quad via vertex_index), instance slot via storage binding
  compact_index (D17), uv_rect atlas support (D15/D18)
- core/particles.rs: ParticlePoolConfig, BlendingMode, ParticleDriver trait
  (D3), ParticlePool (4 buffers + pipeline + bind group), default disc
  texture (D11), unit tests

Wired:
- Scene: SceneGpu keeps queue/sample_count; particle_pools registry +
  create_particle_pool() (default disc when no texture given)
- utils::conf PARTICLE_BILLBOARD_SHADER, module re-exports, prelude
- tests/wgsl_validate.rs: particle billboard naga validation (2 entry points)

Docs: DRAFT call-site/tree synced with the final code; AGENTS.md test count
(138) + wgpu 30 API drift gotcha (contents/DeviceExt/ALPHA_BLENDING,
DepthStencilState no Default, NonZero min_binding_size, const Zeroable).

cargo test -p wsg-lib: 138 pass (121 lib + 10 wgsl + 7), 0 warnings.

docs/DRAFT.md

@@ -60,9 +60,9 @@ seront repris **après** le système de particules (phase 7).
 
 ```
 lib/
-├── shaders/
-│   └── particle_billboard.wgsl   # NOUVEAU : vs_main + fs_main (pas de compute)
 └── src/
+    ├── shaders/
+    │   └── particle_billboard.wgsl   # NOUVEAU : vs_main + fs_main (pas de compute)
     ├── utils/
     │   └── conf.rs               # + pub const PARTICLE_BILLBOARD_SHADER (include_str!, pattern existant)
     ├── resources/
@@ -373,8 +373,7 @@ impl Scene {
         // Construire le pool (buffers + pipeline + bind group)
         let gpu = self.gpu();
         let pool = ParticlePool::new(
-            &gpu.device,
-            &gpu.queue,
+            gpu.device.as_ref(),
             gpu.format,
             gpu.sample_count,
             &config,