The group-reap poll folded its deadline arm into the empty-group arm, so a host
event loop blocked past graceMs + CLOSE_REAP_MARGIN_MS would run the overdue
poll before the grace SIGKILL timer: the group is still non-empty, the deadline
has passed, and the shared arm cancelled the never-fired SIGKILL and finalized —
releasing a SIGTERM-ignoring same-group survivor for good. Split the arms: empty
group cancels the moot timer and finalizes; deadline-with-non-empty-group sends
SIGKILL itself (idempotent if the timer already ran) before finalizing. Adds a
regression test that busy-blocks the loop past both timers and asserts the
survivor's heartbeat freezes.
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