Improve MQTT resilience, clarify behavior, and minor UX

Switch to Paho v2 callbacks; add loop_start() and reconnect_delay_set() for auto-reconnect
Rework on_connect/on_disconnect to v2 signatures; handle session_present and reconnection flows
Gate heartbeats with client.is_connected() and add short retry on rc=4 (NO_CONN)
Re-send discovery after reconnect; ensure re-subscription to all topics
Add startup terminal message with ISO timestamp in simclient.py
Docs: update README and Copilot instructions with reconnection/heartbeat guidance and benign rc=4 notes

README.md

@@ -469,6 +469,13 @@ sudo systemctl status mosquitto
 **Try fallback brokers:**
 Edit `.env` and add `MQTT_BROKER_FALLBACKS`
 
+#### Client auto-reconnect and heartbeat behavior (Nov 2025)
+- The MQTT client now uses Paho v2 callbacks and `loop_start()` for automatic reconnection with exponential backoff.
+- All topic subscriptions are restored in `on_connect` and a discovery message is re-sent on reconnect to re-register the client.
+- Heartbeats are sent only when connected; if publish occurs during a brief reconnect window, Paho may return rc=4 (NO_CONN). The client performs a short retry and logs the outcome.
+- Occasional `Heartbeat publish failed with code: 4` after broker restart or transient network hiccups is expected and not dangerous. It indicates "not connected at this instant"; the next heartbeat typically succeeds.
+- When to investigate: repeated rc=4 with no succeeding "Heartbeat sent" entries over multiple intervals.
+
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