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/closesthit.glsl

@@ -0,0 +1,38 @@
+#version 460
+#extension GL_EXT_ray_tracing : enable
+
+// RTVolume closest-hit — shades the committed procedural sphere hit by
+// its surface normal with a fixed sun + ambient, tinted per instance
+// (gl_InstanceCustomIndexEXT). Mirrors closesthit.wgsl on the WebGPU
+// path.
+
+hitAttributeEXT vec2 attribs;
+layout(location = 0) rayPayloadInEXT vec3 hitValue;
+
+const vec3 SUN_DIR_TO_LIGHT = vec3(0.40, 0.85, 0.35);
+const vec3 SUN_COLOR        = vec3(1.20, 1.10, 0.95);
+const vec3 AMBIENT_COLOR    = vec3(0.16, 0.18, 0.24);
+
+vec3 instanceAlbedo(uint i) {
+    uint h = i * 2654435761u;
+    return vec3(
+        0.35 + 0.6 * float((h >>  0) & 255u) / 255.0,
+        0.35 + 0.6 * float((h >>  8) & 255u) / 255.0,
+        0.35 + 0.6 * float((h >> 16) & 255u) / 255.0);
+}
+
+void main() {
+    // Object-space hit point on the unit sphere is its object-space
+    // normal; gl_ObjectToWorldEXT's rotation part takes it to world.
+    vec3 posObj = gl_ObjectRayOriginEXT + gl_ObjectRayDirectionEXT * gl_HitTEXT;
+    vec3 nObj   = normalize(posObj);
+    vec3 nWorld = normalize(gl_ObjectToWorldEXT * vec4(nObj, 0.0));
+
+    vec3  albedo  = instanceAlbedo(uint(gl_InstanceCustomIndexEXT));
+    vec3  viewDir = -gl_WorldRayDirectionEXT;
+    vec3  nFacing = dot(nWorld, viewDir) > 0.0 ? nWorld : -nWorld;
+    vec3  sunDir  = normalize(SUN_DIR_TO_LIGHT);
+    float nDotL   = max(0.0, dot(nFacing, sunDir));
+
+    hitValue = albedo * (AMBIENT_COLOR + SUN_COLOR * nDotL);
+}