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/MqttCrossProtocolTests.cs

@@ -0,0 +1,164 @@
+using NATS.Server.Auth;
+using NATS.Server.Mqtt;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Mqtt.Tests;
+
+/// <summary>
+/// Tests for the MqttNatsClientAdapter and cross-protocol bridging concepts.
+/// Verifies that MQTT connections can participate in the NATS SubList and
+/// that topic/subject translation works end-to-end.
+/// </summary>
+public class MqttCrossProtocolTests
+{
+    [Fact]
+    public void Adapter_implements_INatsClient()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 42);
+
+        adapter.Id.ShouldBe((ulong)42);
+        adapter.Kind.ShouldBe(ClientKind.Client);
+        adapter.ClientOpts.ShouldBeNull();
+    }
+
+    [Fact]
+    public void Adapter_add_and_remove_subscription()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 1);
+        var account = new Account("test");
+        adapter.Account = account;
+
+        // Add subscription
+        var sub = adapter.AddSubscription("sensor.temp", "sid1");
+        sub.Subject.ShouldBe("sensor.temp");
+        sub.Client.ShouldBe(adapter);
+        adapter.Subscriptions.Count.ShouldBe(1);
+
+        // Verify it's in the SubList
+        var result = account.SubList.Match("sensor.temp");
+        result.PlainSubs.ShouldContain(s => s.Sid == "sid1");
+
+        // Remove subscription
+        adapter.RemoveSubscription("sid1");
+        adapter.Subscriptions.Count.ShouldBe(0);
+
+        // Verify removed from SubList
+        result = account.SubList.Match("sensor.temp");
+        result.PlainSubs.ShouldNotContain(s => s.Sid == "sid1");
+    }
+
+    [Fact]
+    public void Adapter_remove_all_subscriptions()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 1);
+        var account = new Account("test");
+        adapter.Account = account;
+
+        adapter.AddSubscription("a.b", "s1");
+        adapter.AddSubscription("c.d", "s2");
+        adapter.AddSubscription("e.f", "s3");
+        adapter.Subscriptions.Count.ShouldBe(3);
+
+        adapter.RemoveAllSubscriptions();
+        adapter.Subscriptions.Count.ShouldBe(0);
+    }
+
+    [Fact]
+    public void Adapter_queue_outbound_is_noop()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 1);
+
+        adapter.QueueOutbound(new byte[] { 1, 2, 3 }).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Adapter_signal_flush_is_noop()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 1);
+
+        // Should not throw
+        adapter.SignalFlush();
+    }
+
+    [Fact]
+    public void Topic_mapper_integration_with_sublist()
+    {
+        var account = new Account("test");
+
+        // Simulate an MQTT client subscribing to "sensor/+"
+        var natsSubject = MqttTopicMapper.MqttToNats("sensor/+");
+        natsSubject.ShouldBe("sensor.*");
+
+        var sub = new Subscription
+        {
+            Subject = natsSubject,
+            Sid = "mqtt-sub-1",
+        };
+        account.SubList.Insert(sub);
+
+        // Simulate a NATS publish to "sensor.temp" — should match
+        var result = account.SubList.Match("sensor.temp");
+        result.PlainSubs.ShouldContain(s => s.Sid == "mqtt-sub-1");
+
+        // "sensor.humidity" should also match
+        result = account.SubList.Match("sensor.humidity");
+        result.PlainSubs.ShouldContain(s => s.Sid == "mqtt-sub-1");
+
+        // "other.temp" should NOT match
+        result = account.SubList.Match("other.temp");
+        result.PlainSubs.ShouldNotContain(s => s.Sid == "mqtt-sub-1");
+    }
+
+    [Fact]
+    public void Topic_mapper_multilevel_wildcard_with_sublist()
+    {
+        var account = new Account("test");
+
+        // MQTT subscribe to "home/#"
+        var natsSubject = MqttTopicMapper.MqttToNats("home/#");
+        natsSubject.ShouldBe("home.>");
+
+        var sub = new Subscription
+        {
+            Subject = natsSubject,
+            Sid = "mqtt-sub-2",
+        };
+        account.SubList.Insert(sub);
+
+        // Should match multi-level subjects
+        account.SubList.Match("home.living.light").PlainSubs
+            .ShouldContain(s => s.Sid == "mqtt-sub-2");
+        account.SubList.Match("home.kitchen").PlainSubs
+            .ShouldContain(s => s.Sid == "mqtt-sub-2");
+    }
+
+    [Fact]
+    public void Adapter_mqtt_client_id_exposed()
+    {
+        using var stream = new MemoryStream();
+        var listener = CreateTestListener();
+        var connection = new MqttConnection(stream, listener);
+        var adapter = new MqttNatsClientAdapter(connection, 1);
+
+        // ClientId comes from the underlying connection
+        adapter.MqttClientId.ShouldBe(string.Empty); // not yet connected
+    }
+
+    private static MqttListener CreateTestListener()
+        => new("127.0.0.1", 0);
+}