The waiter cleanup pair (asyncTasksByWaiter.remove + rendezvous.close) was
duplicated: two sites sit in a finally, the third was hand-rolled inline
before answer()'s early STALE_TURN return, structurally outside any finally.
Both rendezvous.answerAsk and clearAsyncQuestion(turnId, false) are total
(cannot throw), so the gap never leaked in practice. But the duplicate was
untested: mutating it away left all 1765 tests green, while the two
finally-protected sites are each killed by 8-36 tests. Same shape as #572.
Fix: widen the try to wrap the Task registration and the STALE_TURN check,
so the single finally covers every exit and the hand-rolled copy is gone.
Added a race hook + regression test that deterministically reproduces the
'ask lapsed between the lookup and the unblock' case and proves the fix
still returns STALE_TURN and cleans up exactly once.
Dai Ha
b091c51eee