gpu.py: darwin branch via system_profiler SPDisplaysDataType for GPU
model/VRAM on macOS, falls back to rocm-smi on Linux.
hardware.py: _get_vram_darwin() parses system_profiler output.
node_agent/agent.py: get_load() reports VRAM on darwin via
system_profiler.
tests: 5 new gpu tests (linux/darwin/absent), 3 new hardware tests
(darwin assessment + VRAM parsing).
gramps