feat(webgpu-rt): add intersection stage, procedural hit group, AABB BLAS API

Extends the cross-backend RT type surface for procedural geometry +
any-hit on the WebGPU path:

- RTShaderGroupType::ProceduralHitGroup + RTShaderGroup::intersectionShader
  (mirror VK_RAY_TRACING_SHADER_GROUP_TYPE_PROCEDURAL_HIT_GROUP_KHR).
- WebGPURTStage::Intersection for AABB intersection shaders.
- Mesh::BuildProcedural(span<RTAabb>, opaque) — the WebGPU analog of a
  VK_GEOMETRY_TYPE_AABBS_KHR geometry.
- wgpuRegisterMeshBLAS gains geomType / opaqueFlag / primCount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

interfaces/Crafter.Graphics-ShaderBindingTableWebGPU.cppm

@@ -14,15 +14,23 @@ import std;
 
 export namespace Crafter {
     enum class WebGPURTStage : std::uint8_t {
-        Raygen     = 0,
-        Miss       = 1,
-        ClosestHit = 2,
-        AnyHit     = 3,
+        Raygen       = 0,
+        Miss         = 1,
+        ClosestHit   = 2,
+        AnyHit       = 3,
         // Wavefront RESOLVE-stage tonemap/output hook. Optional: if no
         // Resolve shader is registered, RESOLVE writes the linear accum
         // buffer through unchanged. Signature:
         //   fn <entryFn>(coord: vec2<u32>, hdr: vec4<f32>) -> vec4<f32>
-        Resolve    = 4,
+        Resolve      = 4,
+        // Intersection shader for AABB (procedural) geometry. Run for each
+        // AABB primitive the ray enters during TRACE; reports whether the
+        // procedural surface is hit and at what distance. Signature:
+        //   fn <entryFn>(ray: RayDesc, aabbMin: vec3<f32>, aabbMax: vec3<f32>,
+        //                primitiveId: u32) -> IntersectionResult
+        // `ray` is in object space. IntersectionResult{ hit, t, attribs,
+        // hitKind } is declared by the library prelude.
+        Intersection = 5,
     };
 
     // One WGSL shader source + the function name PipelineRTWebGPU should
@@ -35,6 +43,9 @@ export namespace Crafter {
     //   ClosestHit: fn <entryFn>(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>)
     //   AnyHit:     fn <entryFn>(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) -> u32
     //               returns RT_ANYHIT_ACCEPT / RT_ANYHIT_IGNORE / RT_ANYHIT_END_SEARCH.
+    //   Intersection: fn <entryFn>(ray: RayDesc, aabbMin: vec3<f32>, aabbMax: vec3<f32>,
+    //                              primitiveId: u32) -> IntersectionResult
+    //               IntersectionResult{ hit: bool, t: f32, attribs: vec2<f32>, hitKind: u32 }.
     //
     // `RayDesc`, `HitInfo`, the `RT_*` flag/return constants, the `tlas` /
     // BLAS / mesh-record bindings, and the `traceRay` function are all