test(webgpu-rt): RTMultiShadow example exercising N shadow rays/pixel/bounce (#30)

Five pillars, four colored point lights; closest-hit emits one shadow ray
per light from the same invocation (raysPerPixel = 4, maxDepth = 2), so up
to four rays per pixel rtAccumulate in a single SHADE pass — the contention
case #30 exists for. Each pillar casts four separable colored shadows; an
accumulator race or capacity drop shows as flickering dark noise or a
missing shadow color. Two frames a second apart diff at 2 px / 1.85 M
(last-ulp CAS ordering), confirming no lost updates.

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

examples/README.md

@@ -106,3 +106,15 @@ barrier WebGPU only provides between submits (or between passes), never
 within a single compute pass. WebGPU/DOM only; the same chain is wireable
 on Vulkan today via an offscreen HDR heap image + a composite `RenderPass`
 (the present path records passes generically and barriers between them).
+
+### [RTMultiShadow](RTMultiShadow/)
+Multi-light shadowing through the wavefront RT pipeline (issue #30). Five
+pillars on a checkered ground, four colored point lights; the closest-hit
+emits one shadow ray **per light** from the same invocation, so up to four
+rays per pixel resolve in a single SHADE pass. Exercises both halves of
+the >1-ray-per-pixel-per-bounce widening: the atomic `rtAccumulate`
+(per-channel f32 CAS — concurrent same-pixel adds don't race) and
+`RTPass::raysPerPixel` (scales the ray/hit/payload buffers so the
+per-light emits aren't capacity-dropped). Any regression shows up as
+flickering dark noise or a missing shadow color in the overlap regions.
+WebGPU/DOM only.