drainAll used to log nothing on a clean drain — both existing log calls
(drainSnapshot's per-session failure, drainAll's straggler-sweep warning)
sit on abnormal paths, so "drained fine" and "died on the first session"
looked identical: no log line either way.
Add one log.info at the end of drainAll: "drain complete: released=N
abandoned=M (still BUSY at the shutdown deadline)". It fires on the
normal path, including the all-zero case, and folds both drainSnapshot
passes (main snapshot + straggler sweep) into one line.
drainSnapshot now returns a private DrainTally(released, abandoned)
record instead of void, and the private release(paneId, cause) overload
now returns the removed MemberSession (previously void) so drainSnapshot
can read its state at the moment of removal — the same check
logPreservedForShutdown already makes. Both signature changes are
private with a single call site, so the blast radius stays small.
Dai Ha