Introduces a VARIANT=gpu|cpu build arg to the Dockerfile. The cpu
variant uses ubuntu:22.04 (no CUDA base), installs torch+cpu and
onnxruntime (no GPU deps) via a separate pyproject-cpu.toml / uv-cpu.lock,
and is published as :cpu (dev-cpu on the dev branch) via a new
build-cpu CI job. Saves ~2 GB over the default GPU image.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>