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/rayquery_pick.wgsl

@@ -0,0 +1,48 @@
+// rayQuery picking smoke test (WebGPU/DOM software ray-query path).
+//
+// Shoots a single, fully-determined ray at a known TLAS instance through
+// the injected `rayQuery*` shim and records the committed hit into a
+// storage buffer the host reads back. The whole point is to exercise
+// `_rqTraverseTlas`'s TLAS descent for a realistic (< 8193) instance
+// count — the regime where a hardcoded 16384-leaf start makes every node
+// look internal and the pick always misses (issue #25).
+//
+// rayQuery=true ⇒ @group(1) is the reserved RT heap (do NOT declare it).
+// User bindings live at @group(2)+; the optional push uniform at @group(0).
+
+struct PushData {
+    origin: vec3<f32>,
+    _p0:    f32,
+    dir:    vec3<f32>,
+    _p1:    f32,
+};
+@group(0) @binding(0) var<uniform> push : PushData;
+
+struct PickResult {
+    hit:                 u32,   // 1 = committed triangle hit, 0 = miss
+    instanceCustomIndex: u32,
+    primitiveIndex:      u32,
+    tHit:                f32,
+};
+@group(2) @binding(0) var<storage, read_write> result : PickResult;
+
+@compute @workgroup_size(1)
+fn main() {
+    var rq: RayQuery;
+    rayQueryInitialize(&rq, 0u, RT_FLAG_OPAQUE, 0xFFu,
+                       push.origin, 0.001, push.dir, 10000.0);
+    // Software shim resolves the whole traversal in one proceed call.
+    while (rayQueryProceed(&rq)) {}
+
+    if (rayQueryGetCommittedIntersectionType(&rq) != RT_INTERSECTION_NONE) {
+        result.hit                 = 1u;
+        result.instanceCustomIndex = rayQueryGetCommittedInstanceCustomIndex(&rq);
+        result.primitiveIndex      = rayQueryGetCommittedPrimitiveIndex(&rq);
+        result.tHit                = rayQueryGetCommittedT(&rq);
+    } else {
+        result.hit                 = 0u;
+        result.instanceCustomIndex = 0xFFFFFFFFu;
+        result.primitiveIndex      = 0xFFFFFFFFu;
+        result.tHit                = -1.0;
+    }
+}