Function join_held_worker 

pub(crate) fn join_held_worker(slot: &mut Option<JoinHandle<()>>, inner: &Inner)

Join the worker slot holds, having set the external stop INSIDE the caller’s lock immediately before join() — the set-before-join ordering the lost-stop invariant rests on — and clear capture_tid so a later start sees no stale owner. Every stop path ends here.