v5.6.1-gpu.4

PhysX 5.6.1 GPU build .4

First release whose Linux artifacts contain actual libraries. The .1 to .3
Linux archives shipped 609 KB of headers and documentation: the packaging
step globbed physx/bin/linux.clang, a directory that never existed, and
swallowed the failure with 2>/dev/null || true.

Linux now mirrors Windows: GPU projects enabled, shared libraries, the CUDA
arch list rewritten at its source so CUDA 12.5 is not asked for Blackwell,
the OpenGL headers cudaGL.h needs, and hard gates on libPhysX_64.so and
libPhysXGpu_64.so so an empty archive can no longer be published.

Validated green in run 31712364659 before tagging.