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

interfaces/Crafter.Graphics-RT.cppm

@@ -60,18 +60,26 @@ export namespace Crafter {
     inline constexpr std::uint8_t kRTGeometryInstanceForceOpaque               = 0x4;
     inline constexpr std::uint8_t kRTGeometryInstanceForceNoOpaque             = 0x8;
 
-    // Hit-group identification. Matches VkRayTracingShaderGroupTypeKHR for
+    // Hit-group identification. Matches VkRayTracingShaderGroupTypeKHR.
-    // the two types we actually support (general + triangles-hit).
+    //   General           — raygen / miss / callable
+    //   TrianglesHitGroup — closest-hit/any-hit over triangle geometry
+    //   ProceduralHitGroup — closest-hit/any-hit + an intersection shader
+    //                        over AABB (VK_GEOMETRY_TYPE_AABBS_KHR) geometry
     enum class RTShaderGroupType : std::uint8_t {
         General            = 0,   // raygen / miss / callable
         TrianglesHitGroup  = 1,
+        ProceduralHitGroup = 2,
     };
 
     // Cross-backend description of one entry in the shader-group array
     // passed to PipelineRT::Init. Mirrors the meaningful subset of
     // VkRayTracingShaderGroupCreateInfoKHR: per group, the type and the
     // indices (into the SBT's shader array) for general / closestHit /
-    // anyHit, with kRTShaderUnused == VK_SHADER_UNUSED_KHR for "none".
+    // anyHit / intersection, with kRTShaderUnused == VK_SHADER_UNUSED_KHR
+    // for "none". `intersectionShader` is only consulted for
+    // ProceduralHitGroup; it names the shader run for each AABB the ray
+    // enters (the analog of VkRayTracingShaderGroupCreateInfoKHR::
+    // intersectionShader).
     inline constexpr std::uint32_t kRTShaderUnused = 0xFFFFFFFFu;
 
     struct RTShaderGroup {
@@ -79,5 +87,6 @@ export namespace Crafter {
         std::uint32_t      generalShader      = kRTShaderUnused;
         std::uint32_t      closestHitShader   = kRTShaderUnused;
         std::uint32_t      anyHitShader       = kRTShaderUnused;
+        std::uint32_t      intersectionShader = kRTShaderUnused;
     };
 }

interfaces/Crafter.Graphics-ShaderBindingTableWebGPU.cppm

@@ -23,6 +23,14 @@ export namespace Crafter {
         // buffer through unchanged. Signature:
         //   fn <entryFn>(coord: vec2<u32>, hdr: vec4<f32>) -> vec4<f32>
         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

interfaces/Crafter.Graphics-WebGPU.cppm

@@ -167,6 +167,12 @@ namespace Crafter::WebGPU {
     // that gets appended to a global attribs heap and exposed to RT
     // closest-hit shaders as `vertexAttribs : array<u32>` at
     // @group(1) @binding(7). Pass (nullptr, 0) for positions-only meshes.
+    // `geomType` selects the primitive kind: 0 = triangles (the
+    // verticesPtr/indicesPtr streams), 1 = AABBs (VK_GEOMETRY_TYPE_AABBS) —
+    // then verticesPtr holds 2 vec3 per primitive [min, max], indexCount is
+    // 0, and an intersection shader supplies the hit. `opaqueFlag` is the
+    // geometry's opaque bit (0 lets any-hit run). `primCount` is the
+    // triangle / AABB primitive count.
     __attribute__((import_module("env"), import_name("wgpuRegisterMeshBLAS")))
     extern "C" std::uint32_t wgpuRegisterMeshBLAS(
         float minX, float minY, float minZ,
@@ -175,7 +181,8 @@ namespace Crafter::WebGPU {
         const void* indicesPtr,  std::int32_t indexCount,
         const void* bvhNodesPtr, std::int32_t bvhNodeCount,
         const void* primRemapPtr, std::int32_t primRemapCount,
-        const void* attribsPtr,  std::int32_t attribsByteCount);
+        const void* attribsPtr,  std::int32_t attribsByteCount,
+        std::int32_t geomType, std::int32_t opaqueFlag, std::int32_t primCount);
 
     // RT pipeline build. The library composes WGSL by concatenating the
     // traversal library, generated hit-group switches, and the user-