Add shadow compute shader (voxelShadowCS.hlsl) that traces rays toward
the sun using DXR inline ray queries (RayQuery<>, SM 6.5). Shadows
modulate voxelRT_ in-place via RWTexture2D (no extra render target).
Key fixes to Phase 6.1 BLAS/TLAS infrastructure:
- Sequential index buffer required: Wicked treats IndexCount=0 with
non-null IndexBuffer as "0 indexed triangles" → empty BLAS
- Memory barriers between BLAS→TLAS→RT: without GPUBarrier::Memory()
the TLAS build races with BLAS builds, causing zero ray hits
- inverseViewProjection added to VoxelCB for depth reconstruction
F5 toggles shadows OFF→ON→DEBUG (red=hit, green=miss, blue=backface).
