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/RTMultiShadow/resolve.wgsl

@@ -0,0 +1,7 @@
+// RTMultiShadow RESOLVE-stage tonemap: Reinhard + gamma 2.2 over the
+// linear accumulator. Registered as a WebGPURTStage::Resolve shader.
+fn resolve_main(coord: vec2<u32>, hdr: vec4<f32>) -> vec4<f32> {
+    let mapped = hdr.rgb / (hdr.rgb + vec3<f32>(1.0));
+    let g      = pow(mapped, vec3<f32>(1.0 / 2.2));
+    return vec4<f32>(g, 1.0);
+}