ci: consolidate the GPU build onto macos-port

macos-port and macos-gpu differed only by the GPU workflow file; all
engine/lobby code was identical and macos-gpu just mirrored every
macos-port fix. Adopt macos-build-gpu.yml onto macos-port so both the
compat (llvmpipe, macos-15) and GPU (Mesa 26.2 KosmicKrisp, macos-26)
builds live on one branch and fixes are committed once. The GPU workflow
is workflow_dispatch-only since its from-source toolchain build is heavy.

Author: Mearman