test(bench): SponzaBench harness + #40→HEAD perf measurement (#155)

Headless benchmark around the native Sponza RT scene: times setup and a
measured Render() loop over the full multi-mesh atrium, prints BENCH
metrics, and exits. Includes run-bench.sh and a README documenting the
methodology and the measured net gain from #40 to current master.

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

examples/SponzaBench/closesthit.glsl

@@ -0,0 +1,16 @@
+#version 460
+#extension GL_EXT_ray_tracing : enable
+
+// Benchmark closest-hit: shade from the hit's barycentric coordinates.
+// SponzaBench deliberately avoids the descriptor-heap texture sample the
+// interactive Sponza example uses — the point here is to measure the
+// ray-tracing traversal + frame-loop cost, not texturing, and a
+// texture-free hit keeps the host setup portable across library
+// revisions (issue #155).
+hitAttributeEXT vec2 hitAttrs;
+layout(location = 0) rayPayloadInEXT vec3 hitValue;
+
+void main() {
+    vec3 bary = vec3(1.0 - hitAttrs.x - hitAttrs.y, hitAttrs.x, hitAttrs.y);
+    hitValue = bary;
+}