QoS 1: deliver message then send PUBACK. QoS 2: store in-memory
pending PUBREL, send PUBREC; on PUBREL deliver and send PUBCOMP.
Wire all four PI-packet dispatches (PUBACK/PUBREC/PUBREL/PUBCOMP)
in parser. Add QoS2Pending dictionary to MqttHandler for in-memory
QoS 2 tracking. 6 new tests for QoS 1/2 flows including full
QoS 2 handshake.
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.
