pub fn ensure_global_venv(bootstrap_python: &str) -> Result<PathBuf>Expand description
Create the managed global venv at ~/.mlstack/global/ if it does not yet
exist, using bootstrap_python (a discovered system/uv interpreter) as the
base. Returns the path to the global venv’s python.
Prefers uv venv when available (fast, deterministic); falls back to
python -m venv. This is the deterministic anchor that replaces the old
non-deterministic interpreter scan for global installs.