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.

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttQoSTrackingTests.cs

@@ -43,19 +43,19 @@ public sealed class MqttQoSTrackingTests
         tracker.PendingCount.ShouldBe(1);
         var removed = tracker.Acknowledge(id);
 
-        removed.ShouldBeTrue();
+        removed.ShouldNotBeNull();
         tracker.PendingCount.ShouldBe(0);
     }
 
     [Fact]
-    public void Acknowledge_returns_false_for_unknown()
+    public void Acknowledge_returns_null_for_unknown()
     {
         // Go reference: server/mqtt.go — PUBACK for unknown packet ID is silently ignored
         var tracker = new MqttQoS1Tracker();
 
         var result = tracker.Acknowledge(9999);
 
-        result.ShouldBeFalse();
+        result.ShouldBeNull();
     }
 
     [Fact]