WebGPU RT: wavefront tracer core (GENERATE/PREP/TRACE/SHADE/RESOLVE)

Replace the megakernel @compute entry with five wavefront kernels sharing
one module, connected by GPU ray/hit/payload buffers and a GPU-driven
indirect bounce loop:

  GENERATE -> (PREP -> TRACE -> SHADE) x maxDepth -> RESOLVE

- TRACE contains zero user code (pure _rtwTraverseTlas/Blas, opaque-only).
- PREP publishes dispatchWorkgroupsIndirect args from the live ray count;
  the indirect-args buffer lives in its own bind group so it is never
  bound read-write in the same dispatch that consumes it as INDIRECT.
- New emit/accumulate API: rtEmitPrimaryRay / rtEmitRay / rtAccumulate,
  plus an optional user Resolve stage (tonemap hook; identity by default).
- Per-pass WfParams via a dynamic-offset uniform ring (curIsA/bounce vary
  between passes within one submit).
- Payload-typed wfPayload binding emitted in the codegen region after the
  user's struct Payload; payload travels with each ray (2*W*H slots).
- Request maxBufferSize / maxStorageBufferBindingSize / maxComputeWorkgroups
  PerDimension so the W*H-sized work buffers fit past the 128MB baseline.

VulkanTriangle ported to the new API and renders bit-identical to the
megakernel baseline at maxDepth=1.

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

examples/VulkanTriangle/closesthit.wgsl

@@ -1,6 +1,9 @@
-// WebGPU port of closesthit.glsl. Library concatenates this BEFORE the
-// library helpers, so `Payload` declared here is visible to traceRay,
-// runClosestHit, the mega-switch, and the user's raygen source.
+// Payload declared here so the WGSL assembler sees it before the wfPayload
+// binding, the SHADE dispatch, and the raygen source.
+//
+// Wavefront model: closesthit_main runs in SHADE and accumulates the
+// pixel's color directly (rtAccumulate) instead of writing a payload that
+// raygen reads back.
 
 struct Payload {
     color: vec3<f32>,
@@ -8,5 +11,5 @@ struct Payload {
 
 fn closesthit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) {
     let bary = vec3<f32>(1.0 - hit.attribs.x - hit.attribs.y, hit.attribs.x, hit.attribs.y);
-    (*payload).color = bary;
+    rtAccumulate(bary);
 }