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/README.md

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+# RTVolume
+
+WebGPU software ray tracing of **procedural (AABB) geometry** with an
+**any-hit** cut-out — the two features added for issue #13.
+
+A 3×3×3 grid of unit boxes is registered as an AABB BLAS
+(`Mesh::BuildProcedural`, the WebGPU analog of `VK_GEOMETRY_TYPE_AABBS_KHR`).
+The hit group is a `RTShaderGroupType::ProceduralHitGroup` carrying:
+
+- `intersection.wgsl` — analytic ray–sphere test that turns each box into a
+  radius-1 sphere (runs in TRACE, once per box the ray enters);
+- `anyhit.wgsl` — returns `RT_ANYHIT_IGNORE` for half the cells of a
+  spherical checkerboard, so the ray passes through and the background /
+  spheres behind show through (the visible proof any-hit runs);
+- `closesthit.wgsl` — normal-based Lambert shading, tinted per instance.
+
+The geometry is registered **non-opaque** and the instances clear their
+force-opaque flag, which is what lets the any-hit shader run. Flip the
+instance flag to `kRTGeometryInstanceForceOpaque` (or build the mesh with
+`opaque = true`) to skip any-hit and see solid spheres.
+
+WebGPU/DOM only:
+
+```
+crafter-build --target=wasm32-wasip1 -r
+```