* feat(server)!: add --gpu to choose the GPU on multi-GPU machines
--gpu takes a GPU UUID (as nvidia-smi -L prints) or an nvidia-smi index.
It is applied as CUDA_VISIBLE_DEVICES in the parent before the workers spawn, so the engine still binds cuda:<rank>.
/v1/stats reports the engine's GPU under "gpus".
ft bench bw takes the same --gpu and writes one profile per GPU (benchbw/<gpu-uuid>.json); the legacy benchbw.json is still read by GPU name.
The daemon's /bench/profile returns the running serve's GPU profile.
BREAKING CHANGE: ft checkpoint --device is removed; use --gpu.
* refactor!: resolve --gpu via NVML and bind by UUID, not CUDA_VISIBLE_DEVICES
BREAKING CHANGE: ft bench bw --device is removed; use --gpu.
* refactor: name the id namespaces and drop the multi-device e4m3 scan
Splits the published id into _assigned_physical (UUID) and
_assigned_visible (CUDA ordinal). One process runs on one GPU, so
e4m3_native() judges that card instead of scanning every visible device.
Xiaoze Fan
2757bb5f91