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/miss.glsl

@@ -0,0 +1,11 @@
+#version 460
+#extension GL_EXT_ray_tracing : enable
+
+layout(location = 0) rayPayloadInEXT vec3 hitValue;
+
+void main() {
+    // Soft sky gradient based on ray direction Y. The actual ray dir
+    // isn't accessible without an extra payload field; use a flat warm
+    // tone that matches Sponza's interior lighting.
+    hitValue = vec3(0.10, 0.08, 0.06);
+}