Fold of gate finding (2c Medium regression): raising _INGRESS_DISPATCH_STALL_HEARTBEATS to 4
left the `ingress` row of test_only_pre_handoff_failure_reopens_admission looping 3 heartbeats,
which can never reach the threshold, so its "no healthy-but-deaf report" assertion was disarmed.
Loop the threshold itself so the leg tracks future bumps. Verified: with update_admission's
dispatch counting removed, the heartbeat leg now fails (it passed with range(3)).