Function init_empty_session 

pub fn init_empty_session(
    id: &str,
    explicit_path: Option<&Path>,
    reuse: SessionReuse,
) -> Result<PathBuf, String>

Initialize a NEW, empty session: create its directory, the metrics.db schema, and the invariant session metadata — but record NO execution (nothing has run). Points latest at it. A later run/refine attaches the first execution.

explicit_path overrides the default <sessions-root>/<id> location. reuse governs an already-populated directory.