Cover ClosestHit ray system values: barycentrics, PrimitiveIndex, WorldRay

[MarijnS95]

Depends on #1281

Summary

Three small PSO raytracing tests stacked on #1275, each isolating one shader-observable closest-hit system value from #1268's 👍 list. Same shape as the prior batch in #1277 — one .test file per behavior, single-purpose shader, exact buffer comparison.

• closest-hit-barycentrics.test — 3-lane dispatch, each lane fires at a clearly-interior point of the single triangle so the closest-hit shader reports a known BuiltInTriangleIntersectionAttributes::barycentrics (u, v). Points are picked from the inside of the triangle to avoid the watertight-traversal edge-rule lottery you hit at edge midpoints / vertices (the first cut of this test used midpoint(v0, v1) and one lane silently missed on both backends).
• closest-hit-primitive-index.test — three triangles tiled at x = -3, 0, +3 in a single BLAS. 3-lane dispatch fires straight down at each triangle's centroid; the closest-hit reports PrimitiveIndex() and must match the lane index 0..2.
• closest-hit-world-ray.test — 2-lane dispatch with rays from different z heights (1.0 and 2.0). Closest-hit packs WorldRayOrigin().z, WorldRayDirection().z, and RayTCurrent() through the payload; raygen flattens the float3 into a 6-element Float32 buffer. Verifies the system values match the raygen-side RayDesc and that t is correctly computed by the traversal.

All three are # REQUIRES: raytracing-pipeline with # XFAIL: Clang — clang-dxc doesn't yet lower [shader("…")] entry points. With the Metal RT bring-up in #1281 rebased underneath this branch, all three pass natively on Apple Silicon and Metal is dropped from the XFAIL list.

Test plan

Local on an NVIDIA RTX 3060:

• Linux Vulkan (native offloader)
• Linux D3D12 (Wine + vkd3d-proton + cross-compiled offloader.exe)
• Windows Vulkan (native offloader.exe)
• Windows D3D12 (native offloader.exe)

CI (RT-capable runners):

• windows-nvidia D3D12 (RaytracingTier 1.2)
• windows-intel VK (VK_KHR_ray_tracing_pipeline)
• macOS Metal (supportsRaytracing)