Vendored DS4 Metal boundary
The Metal bridge and GPU declarations track the DS4 boundary audited at commit
ec7642cdd9ec81d01ad4b1fd8f8a3d1511533748 (2026-08-31):
ds4_metal.mds4_gpu.h
The matching Metal kernels, including glm53_*.metal, live in the
repository-level metal/ directory. The other headers remain DS4Server-owned
interfaces and are not copies of DS4 application code.
DS4Server builds and bundles this local snapshot; it does not read a sibling
DS4 checkout. Rust owns the model, graph, session, sampling, and lifecycle.
Objective-C remains only at the platform Metal boundary.