feat(dcl): MxGateway StreamAlarms adapter (snapshot + live transitions, reconnecting)

Adds IAlarmSubscribableConnection to MxGatewayDataConnection (shared session-less
feed, ref-counted), IMxGatewayClient.RunAlarmStreamAsync over the package
StreamAlarmsAsync with internal reconnect, and MxGatewayAlarmMapper
(AlarmFeedMessage/OnAlarmTransitionEvent -> NativeAlarmTransition). Behavior
verified against a live gateway in Task 28; mapper unit-tested.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/MxGatewayDataConnection.cs

@@ -20,7 +20,7 @@ namespace ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Adapters;
 /// <see cref="Disconnected"/>, the actor disposes this adapter, creates a fresh one,
 /// reconnects and re-subscribes all tags.
 /// </summary>
-public class MxGatewayDataConnection : IDataConnection, IBrowsableDataConnection
+public class MxGatewayDataConnection : IDataConnection, IBrowsableDataConnection, IAlarmSubscribableConnection
 {
     private readonly IMxGatewayClientFactory _clientFactory;
     private readonly ILogger<MxGatewayDataConnection> _logger;
@@ -28,6 +28,15 @@ public class MxGatewayDataConnection : IDataConnection, IBrowsableDataConnection
     private ConnectionHealth _status = ConnectionHealth.Disconnected;
     private CancellationTokenSource? _eventLoopCts;
 
+    // Native alarm feed: the gateway StreamAlarms RPC is session-less and
+    // gateway-wide, so one shared feed serves the whole connection. The
+    // DataConnectionActor routes transitions to instances by source reference,
+    // so a single shared callback (the first registered) suffices; subscriptions
+    // are ref-counted so the feed stops when the last one is removed.
+    private CancellationTokenSource? _alarmCts;
+    private int _alarmSubCount;
+    private readonly object _alarmLock = new();
+
     // subscriptionId → (tagPath, callback) so the event loop can route updates by tag,
     // plus tagPath → subscriptionId for reverse lookup. Concurrent because the event
     // loop reads from a background thread while Subscribe/Unsubscribe mutate.
@@ -112,6 +121,13 @@ public class MxGatewayDataConnection : IDataConnection, IBrowsableDataConnection
     public async Task DisconnectAsync(CancellationToken cancellationToken = default)
     {
         _eventLoopCts?.Cancel();
+        lock (_alarmLock)
+        {
+            _alarmCts?.Cancel();
+            _alarmCts?.Dispose();
+            _alarmCts = null;
+            _alarmSubCount = 0;
+        }
         if (_client is not null)
             await _client.DisconnectAsync(cancellationToken);
         _status = ConnectionHealth.Disconnected;
@@ -134,6 +150,43 @@ public class MxGatewayDataConnection : IDataConnection, IBrowsableDataConnection
         await _client!.UnsubscribeAsync(subscriptionId, cancellationToken);
     }
 
+    /// <inheritdoc />
+    public Task<string> SubscribeAlarmsAsync(
+        string sourceReference, string? conditionFilter,
+        AlarmTransitionCallback callback, CancellationToken cancellationToken = default)
+    {
+        lock (_alarmLock)
+        {
+            _alarmSubCount++;
+            if (_alarmCts == null)
+            {
+                _alarmCts = new CancellationTokenSource();
+                var token = _alarmCts.Token;
+                var client = _client!;
+                // Gateway-wide feed (null prefix); the actor filters per source reference.
+                _ = Task.Run(() => client.RunAlarmStreamAsync(null, t => callback(t), token), token);
+            }
+        }
+        return Task.FromResult(Guid.NewGuid().ToString());
+    }
+
+    /// <inheritdoc />
+    public Task UnsubscribeAlarmsAsync(string subscriptionId, CancellationToken cancellationToken = default)
+    {
+        lock (_alarmLock)
+        {
+            if (_alarmSubCount > 0)
+                _alarmSubCount--;
+            if (_alarmSubCount == 0)
+            {
+                _alarmCts?.Cancel();
+                _alarmCts?.Dispose();
+                _alarmCts = null;
+            }
+        }
+        return Task.CompletedTask;
+    }
+
     /// <inheritdoc />
     public async Task<ReadResult> ReadAsync(string tagPath, CancellationToken cancellationToken = default)
     {