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/raygen.wgsl

@@ -0,0 +1,35 @@
+// RTStress raygen (runs in GENERATE). Host-driven pinhole camera at
+// @group(3) (groups 0..2 are reserved by the wavefront pipeline:
+// 0 = WfParams, 1 = data heaps, 2 = indirect args).
+struct Camera {
+    origin:  vec3<f32>,
+    pad0:    f32,
+    right:   vec3<f32>,
+    tanHalf: f32,
+    up:      vec3<f32>,
+    aspect:  f32,
+    forward: vec3<f32>,
+    pad1:    f32,
+};
+@group(3) @binding(0) var<storage, read> camera : Camera;
+
+fn raygen_main(gid: vec3<u32>) {
+    if (gid.x >= wfParams.surfaceW || gid.y >= wfParams.surfaceH) { return; }
+
+    let pixelf = vec2<f32>(f32(gid.x), f32(gid.y));
+    let res    = vec2<f32>(f32(wfParams.surfaceW), f32(wfParams.surfaceH));
+    let uv     = (pixelf + vec2<f32>(0.5)) / res;
+    let ndc    = uv * 2.0 - vec2<f32>(1.0);
+
+    let direction = normalize(
+        camera.right   * (ndc.x  * camera.aspect * camera.tanHalf) +
+        camera.up      * (-ndc.y * camera.tanHalf) +
+        camera.forward);
+
+    var p: Payload;
+    p.color     = vec3<f32>(0.0);
+    p.shadowRay = 0u;
+
+    rtEmitPrimaryRay(camera.origin, 0.01, direction, 100000.0,
+                     0u, 0xFFu, 0u, 0u, p);
+}