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/anyhit.wgsl

@@ -0,0 +1,24 @@
+// RTVolume any-hit shader (runs in TRACE on every candidate sphere hit,
+// because the geometry is registered non-opaque). Punches a spherical
+// checkerboard of holes: for half the cells it returns RT_ANYHIT_IGNORE,
+// so the ray passes straight through and the background / spheres behind
+// show through. Returning RT_ANYHIT_ACCEPT keeps the hit. This is the
+// visible proof the any-hit path runs — with it the spheres are perforated,
+// without it they would be solid.
+fn anyhit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) -> u32 {
+    // Object-space hit point on the unit sphere → its normal/direction.
+    let posObj = hit.objectRayOrigin + hit.objectRayDirection * hit.t;
+    let n = normalize(posObj);
+
+    let PI = 3.14159265;
+    let longitude = atan2(n.z, n.x);                 // [-PI, PI]
+    let latitude  = asin(clamp(n.y, -1.0, 1.0));     // [-PI/2, PI/2]
+
+    let cu = i32(floor((longitude + PI) / PI * 6.0));
+    let cv = i32(floor((latitude + PI * 0.5) / PI * 6.0));
+
+    if (((cu + cv) & 1) == 0) {
+        return RT_ANYHIT_IGNORE;   // cut-out cell — see through
+    }
+    return RT_ANYHIT_ACCEPT;
+}