docs(webgpu-rt): add RTVolume example (procedural spheres + any-hit cut-out)

A 3x3x3 grid of AABB-geometry spheres rendered through an analytic
ray-sphere intersection shader, with an any-hit spherical-checkerboard
cut-out so the background shows through. Exercises both features end to
end on the WebGPU wavefront tracer.

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

examples/RTVolume/closesthit.wgsl

@@ -0,0 +1,37 @@
+// RTVolume closest-hit (runs in SHADE). Shades the procedural sphere by
+// its surface normal with a fixed sun + ambient, tinted per instance.
+//
+// Payload declared here so the assembler sees it before wfPayload / SHADE.
+struct Payload {
+    color: vec3<f32>,
+};
+
+const SUN_DIR_TO_LIGHT: vec3<f32> = vec3<f32>(0.40, 0.85, 0.35);
+const SUN_COLOR:        vec3<f32> = vec3<f32>(1.20, 1.10, 0.95);
+const AMBIENT_COLOR:    vec3<f32> = vec3<f32>(0.16, 0.18, 0.24);
+
+fn instanceAlbedo(i: u32) -> vec3<f32> {
+    let h = i * 2654435761u;
+    return vec3<f32>(
+        0.35 + 0.6 * f32((h >>  0u) & 255u) / 255.0,
+        0.35 + 0.6 * f32((h >>  8u) & 255u) / 255.0,
+        0.35 + 0.6 * f32((h >> 16u) & 255u) / 255.0);
+}
+
+fn closesthit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) {
+    // Object-space hit point on the unit sphere is its object-space normal.
+    let posObj = hit.objectRayOrigin + hit.objectRayDirection * hit.t;
+    let nObj   = normalize(posObj);
+    let nWorld = normalize(vec3<f32>(
+        dot(hit.objectToWorldR0.xyz, nObj),
+        dot(hit.objectToWorldR1.xyz, nObj),
+        dot(hit.objectToWorldR2.xyz, nObj)));
+
+    let albedo  = instanceAlbedo(hit.customIndex);
+    let viewDir = -ray.direction;
+    let nFacing = select(-nWorld, nWorld, dot(nWorld, viewDir) > 0.0);
+    let sunDir  = normalize(SUN_DIR_TO_LIGHT);
+    let nDotL   = max(0.0, dot(nFacing, sunDir));
+
+    rtAccumulate(albedo * (AMBIENT_COLOR + SUN_COLOR * nDotL));
+}