feat(webgpu-rt): atomic pixel accumulator + raysPerPixel for >1 ray/pixel/bounce (#30)

wfAccum becomes array<atomic<u32>> (4 slots/pixel, f32 bit patterns — same
16 B/pixel footprint) and rtAccumulate CASes each channel, so N rays for one
pixel may resolve in the same SHADE pass without the read-modify-write
racing. GENERATE clears with atomicStore (bitcast(0.0) == 0u); RESOLVE
atomicLoads + bitcasts the vec4 it hands runResolve.

Capacity half: RTPass::raysPerPixel (default 1) scales the wavefront
ray/hit/payload buffers to raysPerPixel·W·H rays per bounce so the
per-light emits actually fit instead of being dropped by rtEmitRay's
capacity guard. The accumulator stays per-pixel.

No flag-gating: uncontended the CAS succeeds first try — RTStress SHADE
stays at its documented ~1.0 ms and master-vs-branch renders are
byte-identical, so single-ray consumers pay nothing.

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

README.md

@@ -101,7 +101,9 @@ shaded through an intersection shader with an any-hit cut-out.
   bounce loop (`dispatchWorkgroupsIndirect`). TRACE carries zero user
   code (traversal + intersection only); user raygen calls
   `rtEmitPrimaryRay`, and closesthit / miss run in SHADE where they
-  `rtEmitRay` continuation/shadow rays and `rtAccumulate` radiance. An
+  `rtEmitRay` continuation/shadow rays and `rtAccumulate` radiance
+  (atomic — any number of rays per pixel per bounce, e.g. one shadow
+  ray per light). An
   optional Resolve shader tonemaps the linear accumulator. See
   [WAVEFRONT-DESIGN.md](WAVEFRONT-DESIGN.md).
 - **ComputeShader / WebGPUComputeShader** — Tier 1 wrapper used by the