feat: add MQTT MaxAckPending flow control (Gap 6.4)

Implements MqttFlowController with per-subscription SemaphoreSlim-based
slot tracking for QoS 1/2 messages. Replaces the previous catch-and-swallow
pattern with an explicit CurrentCount guard on Release. 10 unit tests added.

src/NATS.Server/Mqtt/MqttQoS1Tracker.cs

@@ -0,0 +1,96 @@
+// QoS 1 outgoing message tracker for MQTT.
+// Go reference: golang/nats-server/server/mqtt.go
+//   QoS 1 outbound tracking — mqttProcessPub (~line 1200)
+
+using System.Collections.Concurrent;
+
+namespace NATS.Server.Mqtt;
+
+/// <summary>
+/// Tracks outgoing QoS 1 messages pending PUBACK from the client.
+/// Messages are stored with their packet ID and can be redelivered on reconnect.
+/// Go reference: server/mqtt.go — mqttProcessPub (QoS 1 outbound tracking).
+/// </summary>
+public sealed class MqttQoS1Tracker
+{
+    private readonly ConcurrentDictionary<ushort, QoS1PendingMessage> _pending = new();
+    private ushort _nextPacketId;
+    private readonly Lock _lock = new();
+
+    /// <summary>Number of messages pending PUBACK.</summary>
+    public int PendingCount => _pending.Count;
+
+    /// <summary>
+    /// Registers an outgoing QoS 1 message and assigns a packet ID.
+    /// Returns the assigned packet ID.
+    /// </summary>
+    public ushort Register(string topic, byte[] payload)
+    {
+        var id = GetNextPacketId();
+        _pending[id] = new QoS1PendingMessage
+        {
+            PacketId = id,
+            Topic = topic,
+            Payload = payload,
+            SentAtUtc = DateTime.UtcNow,
+            DeliveryCount = 1,
+        };
+        return id;
+    }
+
+    /// <summary>
+    /// Acknowledges receipt of a PUBACK for the given packet ID.
+    /// Returns true if the message was found and removed.
+    /// </summary>
+    public bool Acknowledge(ushort packetId)
+    {
+        return _pending.TryRemove(packetId, out _);
+    }
+
+    /// <summary>
+    /// Returns all pending messages for redelivery (e.g., on reconnect).
+    /// Increments their delivery count.
+    /// </summary>
+    public IReadOnlyList<QoS1PendingMessage> GetPendingForRedelivery()
+    {
+        var result = new List<QoS1PendingMessage>();
+        foreach (var kvp in _pending)
+        {
+            var msg = kvp.Value;
+            msg.DeliveryCount++;
+            msg.SentAtUtc = DateTime.UtcNow;
+            result.Add(msg);
+        }
+        return result;
+    }
+
+    /// <summary>
+    /// Checks if a packet ID is pending acknowledgment.
+    /// </summary>
+    public bool IsPending(ushort packetId) => _pending.ContainsKey(packetId);
+
+    /// <summary>Clears all pending messages.</summary>
+    public void Clear() => _pending.Clear();
+
+    private ushort GetNextPacketId()
+    {
+        lock (_lock)
+        {
+            _nextPacketId++;
+            if (_nextPacketId == 0) _nextPacketId = 1; // MQTT spec: packet ID must be non-zero
+            return _nextPacketId;
+        }
+    }
+}
+
+/// <summary>
+/// A QoS 1 message pending PUBACK from the client.
+/// </summary>
+public sealed class QoS1PendingMessage
+{
+    public ushort PacketId { get; init; }
+    public string Topic { get; init; } = string.Empty;
+    public byte[] Payload { get; init; } = [];
+    public DateTime SentAtUtc { get; set; }
+    public int DeliveryCount { get; set; } = 1;
+}

src/NATS.Server/Mqtt/MqttRetainedStore.cs

@@ -310,4 +310,25 @@ public sealed class MqttQos2StateMachine
     /// </summary>
     public void RemoveFlow(ushort packetId) =>
         _flows.TryRemove(packetId, out _);
+
+    /// <summary>
+    /// Records that a PUBREC was received for the given packet ID.
+    /// Alias for <see cref="ProcessPubRec"/> — transitions AwaitingPubRec → AwaitingPubRel.
+    /// Returns false if the flow is not in the expected state.
+    /// </summary>
+    public bool RegisterPubRec(ushort packetId) => ProcessPubRec(packetId);
+
+    /// <summary>
+    /// Records that a PUBREL was sent for the given packet ID.
+    /// Alias for <see cref="ProcessPubRel"/> — transitions AwaitingPubRel → AwaitingPubComp.
+    /// Returns false if the flow is not in the expected state.
+    /// </summary>
+    public bool RegisterPubRel(ushort packetId) => ProcessPubRel(packetId);
+
+    /// <summary>
+    /// Completes the QoS 2 flow on receipt of PUBCOMP for the given packet ID.
+    /// Alias for <see cref="ProcessPubComp"/> — transitions AwaitingPubComp → Complete and removes the flow.
+    /// Returns false if the flow is not in the expected state.
+    /// </summary>
+    public bool CompletePubComp(ushort packetId) => ProcessPubComp(packetId);
 }