Client.await

Pump admitted work until this operation is finished. Only one blocking caller may own the client at a time. Cancellation removes queued work or drains admitted work before raising Cancellation.Cancelled. The handle retains its outcome. Draining has at most one exchange timeout from detection; local scratch filesystem I/O and owned-child cleanup must also finish. Each owned child gets 200 ms of TERM grace before forced termination and reaping; scheduling and kernel completion can add time. No background scheduler or worker is started.