MqttClient is the interface the HA layer will use; MosquittoClient
implements it with one std::thread for DNS, connect, network loop and
reconnect with backoff (D45, amends D42: mosquitto_loop_start() blocks
the caller on DNS and stops silently on TLS/protocol errors).
tst_mqtt tests it against a local mosquitto: messages, retained
messages, first connect and reconnect, last will, login.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
