feat: implement full MQTT Go parity across 5 phases — binary protocol, auth/TLS, cross-protocol bridging, monitoring, and JetStream persistence
Phase 1: Binary MQTT 3.1.1 wire protocol with PipeReader-based parsing,
full packet type dispatch, and MQTT 3.1.1 compliance checks.
Phase 2: Auth pipeline routing MQTT CONNECT through AuthService,
TLS transport with SslStream wrapping, pinned cert validation.
Phase 3: IMessageRouter refactor (NatsClient → INatsClient),
MqttNatsClientAdapter for cross-protocol bridging, MqttTopicMapper
with full Go-parity topic/subject translation.
Phase 4: /connz mqtt_client field population, /varz actual MQTT port.
Phase 5: JetStream persistence — MqttStreamInitializer creates 5
internal streams, MqttConsumerManager for QoS 1/2 consumers,
subject-keyed session/retained lookups replacing linear scans.
All 503 MQTT tests and 1589 Core tests pass.
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