Function ensure_named_venv 

pub fn ensure_named_venv(name: &str, bootstrap_python: &str) -> Result<PathBuf>

Create the managed NAMED env at ~/.mlstack/envs/<name>/ if absent (Tenet 1: install-to-env isolation). Returns the path to its python.

name is validated (non-empty, no path separators / traversal) since it is interpolated into a filesystem path.