Function session_id_from_stem 

pub fn session_id_from_stem(stem: &str) -> &str

The session id embedded in a rollout filename stem (rollout-<timestamp>-<uuid>): the trailing UUID when present, otherwise the whole stem. This is the same fallback identity the reader assigns a session whose file has no session_meta line, so ids derived from filenames agree with Session.id — and either form resolves through PathResolver::find_rollout_file.