Replace per-chunk DrawInstanced loop with a single DrawInstancedIndirectCount.
CPU fills indirect args buffer with same frustum+backface cull logic as Phase 2.1.
Key discoveries:
- Wicked Engine command signature includes push constant (20-byte stride, not 16)
- SV_VertexID does not reliably include startVertexLocation with ExecuteIndirect
- Solution: pack chunkIndex|(faceIndex<<16) in push constant, VS reconstructs
quad offset from GPUChunkInfo lookup
- No explicit DX12 barriers needed (implicit promotion from COMMON suffices)
Also adds voxel_engine_spec.md and updates references from .docx to .md.
