fix(install): detect x86_64 Python venv on Apple Silicon and rebuild as arm64

On Apple Silicon, uv can create the venv from a cached x86_64 (Rosetta)
Python, so the venv reports x86_64 to wheel resolvers and the torch install
never resolves (the CPU index ships no macOS wheels). Detect an x86_64 venv
on an arm64 host and recreate it with an arch-explicit arm64 CPython,
re-inspecting before the existing 3.13.8 check so both invariants hold.
Skipped when --python is set; non-macOS, Intel, and healthy arm64 venvs are
unaffected.

Co-authored-by: Daniel Han <danielhanchen@gmail.com>