[Feature] GPU (CUDA) inference as a cuda extra #164

The engine runs on CPU; Windows and Linux install CPU torch wheels (#139). Add a cuda extra (CUDA torch wheels) and a device option so the engine can run on an NVIDIA GPU (SPEC S1.11; PROJECT §7 M4).

Done when: uv sync --extra cuda installs CUDA torch and the engine runs on cuda with the same tests passing on a GPU runner.