feat(dcl): add StaleTagMonitor for heartbeat-based disconnect detection

Composable StaleTagMonitor class in Commons fires a Stale event when no
value is received within a configurable max silence period. Integrated
into both LmxProxyDataConnection and OpcUaDataConnection adapters via
optional HeartbeatTagPath/HeartbeatMaxSilence connection config keys.
When stale, the adapter fires Disconnected triggering the standard
reconnect cycle. 10 unit tests cover timer behavior.

src/ScadaLink.DataConnectionLayer/Adapters/LmxProxyDataConnection.cs

@@ -30,6 +30,8 @@ public class LmxProxyDataConnection : IDataConnection
 
     private readonly Dictionary<string, ILmxSubscription> _subscriptions = new();
     private volatile bool _disconnectFired;
+    private StaleTagMonitor? _staleMonitor;
+    private string? _heartbeatSubscriptionId;
 
     public LmxProxyDataConnection(ILmxProxyClientFactory clientFactory, ILogger<LmxProxyDataConnection> logger)
     {
@@ -57,10 +59,44 @@ public class LmxProxyDataConnection : IDataConnection
         _disconnectFired = false;
 
         _logger.LogInformation("LmxProxy connected to {Host}:{Port}", _host, _port);
+
+        // Heartbeat stale tag monitoring (optional)
+        await StartHeartbeatMonitorAsync(connectionDetails, cancellationToken);
+    }
+
+    private async Task StartHeartbeatMonitorAsync(IDictionary<string, string> connectionDetails, CancellationToken cancellationToken)
+    {
+        if (!connectionDetails.TryGetValue("HeartbeatTagPath", out var heartbeatTag) || string.IsNullOrWhiteSpace(heartbeatTag))
+            return;
+
+        var maxSilenceSeconds = connectionDetails.TryGetValue("HeartbeatMaxSilence", out var silenceStr)
+            && int.TryParse(silenceStr, out var sec) ? sec : 30;
+
+        _staleMonitor?.Dispose();
+        _staleMonitor = new StaleTagMonitor(TimeSpan.FromSeconds(maxSilenceSeconds));
+        _staleMonitor.Stale += () =>
+        {
+            _logger.LogWarning("LmxProxy heartbeat tag '{Tag}' stale — no update in {Seconds}s", heartbeatTag, maxSilenceSeconds);
+            RaiseDisconnected();
+        };
+
+        try
+        {
+            _heartbeatSubscriptionId = await SubscribeAsync(heartbeatTag, (_, _) => _staleMonitor.OnValueReceived(), cancellationToken);
+            _staleMonitor.Start();
+            _logger.LogInformation("LmxProxy heartbeat monitor started for '{Tag}' with {Seconds}s max silence", heartbeatTag, maxSilenceSeconds);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Failed to subscribe to heartbeat tag '{Tag}' — stale monitor not active", heartbeatTag);
+            _staleMonitor.Dispose();
+            _staleMonitor = null;
+        }
     }
 
     public async Task DisconnectAsync(CancellationToken cancellationToken = default)
     {
+        StopHeartbeatMonitor();
         if (_client != null)
         {
             await _client.DisconnectAsync();
@@ -200,8 +236,16 @@ public class LmxProxyDataConnection : IDataConnection
         }
     }
 
+    private void StopHeartbeatMonitor()
+    {
+        _staleMonitor?.Dispose();
+        _staleMonitor = null;
+        _heartbeatSubscriptionId = null;
+    }
+
     public async ValueTask DisposeAsync()
     {
+        StopHeartbeatMonitor();
         foreach (var subscription in _subscriptions.Values)
         {
             try { await subscription.DisposeAsync(); }