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/raygen.glsl

@@ -0,0 +1,50 @@
+#version 460
+#extension GL_EXT_ray_tracing : enable
+#extension GL_EXT_shader_image_load_formatted : enable
+#extension GL_EXT_shader_explicit_arithmetic_types_int16 : enable
+#extension GL_EXT_descriptor_heap : enable
+#extension GL_EXT_nonuniform_qualifier : enable
+
+// Specialization constant set from descriptorHeap.bufferStartElement —
+// same pattern as the Sponza example. The TLAS lives at descriptor_heap
+// slot `bufferStart`, the per-frame output image at heap slot 0.
+layout(constant_id = 0) const uint16_t bufferStart = 0us;
+layout(descriptor_heap) uniform accelerationStructureEXT topLevelAS[];
+layout(descriptor_heap) uniform writeonly image2D image[];
+
+layout(location = 0) rayPayloadEXT vec3 hitValue;
+
+void main() {
+    uvec2 pixel      = gl_LaunchIDEXT.xy;
+    uvec2 resolution = gl_LaunchSizeEXT.xy;
+    vec2  uv         = (vec2(pixel) + 0.5) / vec2(resolution);
+    vec2  ndc        = uv * 2.0 - 1.0;
+
+    // Fixed camera framing the 3x3x3 grid (extent = (kGrid-1)*kSpacing
+    // = 6): position ext*(1.1, 0.8, 1.6) looking at the grid centre —
+    // the WebGPU example's initial free-camera pose.
+    vec3 origin  = vec3(6.6, 4.8, 9.6);
+    vec3 forward = normalize(-origin);
+    vec3 right   = normalize(cross(forward, vec3(0.0, 1.0, 0.0)));
+    vec3 up      = cross(right, forward);
+
+    float aspect  = float(resolution.x) / float(resolution.y);
+    float tanHalf = tan(radians(70.0) * 0.5);
+    vec3 direction = normalize(
+        right * (ndc.x * aspect * tanHalf) +
+        up    * (-ndc.y * tanHalf) +
+        forward);
+
+    traceRayEXT(
+        topLevelAS[bufferStart],
+        gl_RayFlagsNoneEXT,
+        0xff,
+        0, 0, 0,
+        origin,
+        0.01,
+        direction,
+        100000.0,
+        0);
+
+    imageStore(image[0], ivec2(pixel), vec4(hitValue, 1.0));
+}