Add ParsePub, ParseSubsOrUnsubs, ProcessPub (QoS 0), ProcessSubs, ProcessUnsubs, EnqueueSubAck, and EnqueueUnsubAck to MqttPacketHandlers. Wire PUB/SUB/UNSUB dispatch cases in MqttParser. Add ReadSlice to MqttReader for raw payload extraction. 18 new unit tests covering parsing, flags, error cases, QoS downgrade, and full flow. 1 new integration test verifying SUBSCRIBE handshake over TCP.