test(webgpu-rt): RayQueryPick example exercising the rayQuery TLAS shim (#25)

Adds an 8^3 = 512-instance TLAS pick test that shoots one analytically
determined ray through a rayQuery=true PlainComputeShader and checks the
read-back committed hit (customIndex 484, t 40.75). 512 instances sit in
the < 8193 regime that the hardcoded 16384-leaf start used to miss, so the
example fails fast if the shim regresses. Verified in Firefox/WebGPU:
"[RayQueryPick] PASS".

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

examples/RayQueryPick/README.md

@@ -0,0 +1,35 @@
+# RayQueryPick
+
+Regression test for the WebGPU software **ray-query** shim
+(`additional/dom-webgpu.js`, `_rqTraverseTlas`).
+
+Builds an 8³ = 512-instance TLAS and shoots one fully-determined ray
+through a `rayQuery=true` `PlainComputeShader`. The committed hit is read
+back to the host and checked against the analytically-known answer
+(instance `customIndex = 484`, `t = 40.75`). On success the console prints:
+
+```
+[RayQueryPick] result: hit=1 customIndex=484 prim=6 t=40.75
+[RayQueryPick] PASS — rayQuery TLAS traversal hit the expected instance
+```
+
+## Why 512 instances
+
+The TLAS sweep tree is padded to `next_pow2(instanceCount)` leaves. The
+rayQuery shim used to detect BVH leaves with a hardcoded `16384 - 1` leaf
+start, so for any scene with fewer than 8193 instances **no node index
+ever reached a leaf** and every pick missed (issue #25). 512 sits squarely
+in that broken regime, so this example fails fast if the shim regresses to
+a static leaf start. The shim now derives the leaf start from a per-build
+`RqTlasMeta.nPadded` uniform, matching the megakernel `_rtwTraverseTlas`.
+
+The scene also renders through the wavefront RT pipeline (same as
+RTStress) so the run produces a visible frame, but the pass/fail signal is
+the console line above.
+
+```bash
+cd examples/RayQueryPick
+crafter-build -r --target=wasm32-wasip1
+```
+
+WebGPU/DOM only — the native path uses hardware ray queries.