Old heartbeat trusted the retained zigbee2mqtt/bridge/state topic, which stays
{"state":"online"} even while the container crash-loops (6900+ restarts
went undetected). New logic: RestartCount increase since last run =
authoritative DOWN signal; container running = secondary guard; bridge state
is informational only (z2m publishes it on transition, not continuously, so
missing in a short window is normal).
david