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.

lib/tests/wgsl_validate.rs

@@ -86,6 +86,37 @@ fn gpu_driven_shader_is_valid_wgsl() {
     );
 }
 
+/// Parses and fully validates the embedded `particle_billboard.wgsl` shader (Étape 28 A) via naga.
+/// The particle pool compiles it into one `RenderPipeline` (empty vertex layout — quad via
+/// `@builtin(vertex_index)`, instance slot via storage binding, D17/D19), so this offline
+/// validation is the guarantee of its validity. The contract expects two entry points:
+/// `vs_main` + `fs_main` (no compute in this step).
+#[test]
+fn particle_billboard_shader_is_valid_wgsl() {
+    let src = include_str!("../src/shaders/particle_billboard.wgsl");
+    let module = naga::front::wgsl::parse_str(src)
+        .unwrap_or_else(|e| panic!("particle_billboard.wgsl: parsing error: {e:?}"));
+
+    let mut validator = naga::valid::Validator::new(
+        naga::valid::ValidationFlags::all(),
+        naga::valid::Capabilities::all(),
+    );
+    validator
+        .validate(&module)
+        .unwrap_or_else(|e| panic!("particle_billboard.wgsl: validation failed: {e:?}"));
+
+    let entry_names: Vec<&str> = module
+        .entry_points
+        .iter()
+        .map(|ep| ep.name.as_str())
+        .collect();
+    assert_eq!(
+        entry_names,
+        vec!["vs_main", "fs_main"],
+        "the two entry points (vs + fs) are expected"
+    );
+}
+
 /// Parses and fully validates the embedded `tonemap.wgsl` shader (Étape 20) via naga.
 /// The renderer compiles it into one `RenderPipeline` (vertex `vs_main` + one of the two
 /// fragment entry points `fs_aces` / `fs_reinhard`), so this offline validation is the