Log MQTT packets dropped while a broker is down - #31

Packets heard while an MQTT broker is down were dropped without a word. They are still dropped (no buffer), but now logged: one warning when a broker starts dropping, and broker publishing again dropped=N when it recovers.

Found during the joint on-air test: MQTT rows ran exactly 1 short of recv, because a packet heard in the half-second before the first connect was dropped silently (behaviour since fa0bc45).

Two silent paths, both now counted in doPublish

• Not connected yet / nil client: dropped before this PR too, only into an unread counter.
• Broker lost mid-run: paho's IsConnected() stays true while it auto-reconnects, and a QoS 0 publish then completes its token without being sent or stored. The first version of this fix missed it; running the real binary caught it. Packet rows are now dropped and counted when IsConnectionOpen() is false.
• QoS 1 status publishes are untouched: paho stores those across a reconnect.

Testing

• Unit test with a fake client that is down, reconnecting, or up: one warning per outage, the recovery count, and a second warning on a mid-run outage. Mutation-checked: no reset, warning per packet, no recovery line, and reverting the reconnecting check each fail it.
• Release-style binary against a fake KISS modem feeding a numbered packet every 0.5 s, and a real Mosquitto that starts late, stops mid-run, and returns, with a subscriber capturing rows:

• Full suite under -race.