test(vulkan-rt): port RTVolume example to native Vulkan (#33)

Regression test for the procedural BLAS path: the same 3x3x3 grid of
unit-box AABBs runs through a PROCEDURAL_HIT_GROUP_KHR group whose GLSL
intersection shader (reportIntersectionEXT) turns each box into a
radius-1 sphere, the any-hit shader punches the spherical-checkerboard
cut-out (visible proof non-opaque geometry runs any-hit), and the
closest-hit shades per-instance tints — the WebGPU example behavior
reproduced natively. Fixed camera in raygen.glsl; the WebGPU/DOM path is
unchanged.

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

examples/RTVolume/miss.glsl

@@ -0,0 +1,12 @@
+#version 460
+#extension GL_EXT_ray_tracing : enable
+
+// RTVolume miss — vertical sky gradient, also what shows through the
+// any-hit cut-out cells. Mirrors miss.wgsl on the WebGPU path.
+
+layout(location = 0) rayPayloadInEXT vec3 hitValue;
+
+void main() {
+    float t = clamp(gl_WorldRayDirectionEXT.y * 0.5 + 0.5, 0.0, 1.0);
+    hitValue = mix(vec3(0.05, 0.07, 0.12), vec3(0.45, 0.60, 0.85), t);
+}