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-Mesh.cppm

@@ -87,6 +87,17 @@ export namespace Crafter {
     };
     static_assert(sizeof(BVHNode) == 32);
 
+    // One procedural primitive's axis-aligned bounding box, in object
+    // space. The analog of VkAabbPositionsKHR — the BLAS stores these
+    // instead of triangles and an intersection shader (registered in the
+    // hit group as a ProceduralHitGroup) reports the actual surface hit
+    // for each AABB the ray enters.
+    struct RTAabb {
+        float min[3];
+        float max[3];
+    };
+    static_assert(sizeof(RTAabb) == 24);
+
     class Mesh {
         public:
         // BLAS "handle": opaque identity that goes into
@@ -119,6 +130,19 @@ export namespace Crafter {
         // as `vertexAttribs : array<u32>` with a per-mesh u32-word offset.
         void Build(const ::Crafter::CompressedMeshAsset& asset,
                    WebGPUCommandEncoderRef                cmd = 0);
+
+        // Build an AABB (procedural) BLAS from a list of object-space boxes
+        // — the WebGPU analog of a VK_GEOMETRY_TYPE_AABBS_KHR geometry. The
+        // hit group bound to instances of this mesh must be a
+        // ProceduralHitGroup carrying an intersection shader; that shader is
+        // invoked for each box the ray enters and reports the surface hit.
+        // `opaque` is the geometry's opaque bit: pass false to let any-hit
+        // shaders run (the default for procedural geometry, which is usually
+        // transparent / volumetric). The `cmd` parameter is unused on
+        // WebGPU — kept for API symmetry with the triangle path.
+        void BuildProcedural(std::span<const RTAabb> aabbs,
+                             bool                    opaque = false,
+                             WebGPUCommandEncoderRef cmd    = 0);
     };
 }
 #endif // CRAFTER_GRAPHICS_WINDOW_DOM