feat(mqtt): add MQTT packet parser, dispatch, and ReadLoop integration
Implement Task 2 of MQTT orchestration:
- Create MqttParser.cs with loop-based packet parser and dispatch switch
- Add MqttReader field to MqttHandler for per-connection parsing state
- Wire ReadLoop to call MqttParser for MQTT connections
- Implement PINGREQ handler (enqueues PINGRESP)
- CONNECT-first enforcement (rejects non-CONNECT as first packet)
- Partial packet handling via MqttReader pending buffer
- 13 unit tests covering parser, dispatch, partial packets, and edge cases
- Stub dispatch entries for CONNECT, PUBLISH, SUB, UNSUB, DISCONNECT (NotImplementedException)
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