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.Commons/Types/StaleTagMonitor.cs

@@ -0,0 +1,68 @@
+namespace ScadaLink.Commons.Types;
+
+/// <summary>
+/// Monitors a heartbeat tag subscription for staleness. If no value is received
+/// within <see cref="MaxSilence"/>, the <see cref="Stale"/> event fires.
+/// Composable into any IDataConnection adapter.
+/// </summary>
+public sealed class StaleTagMonitor : IDisposable
+{
+    private readonly TimeSpan _maxSilence;
+    private Timer? _timer;
+    private volatile bool _staleFired;
+
+    public StaleTagMonitor(TimeSpan maxSilence)
+    {
+        if (maxSilence <= TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(maxSilence), "MaxSilence must be positive.");
+        _maxSilence = maxSilence;
+    }
+
+    /// <summary>
+    /// Fires when no value has been received within <see cref="MaxSilence"/>.
+    /// Fires once per stale period — resets after <see cref="OnValueReceived"/> is called.
+    /// </summary>
+    public event Action? Stale;
+
+    public TimeSpan MaxSilence => _maxSilence;
+
+    /// <summary>
+    /// Start monitoring. The timer begins counting from now.
+    /// </summary>
+    public void Start()
+    {
+        _staleFired = false;
+        _timer?.Dispose();
+        _timer = new Timer(OnTimerElapsed, null, _maxSilence, Timeout.InfiniteTimeSpan);
+    }
+
+    /// <summary>
+    /// Signal that a value was received. Resets the stale timer.
+    /// </summary>
+    public void OnValueReceived()
+    {
+        _staleFired = false;
+        _timer?.Change(_maxSilence, Timeout.InfiniteTimeSpan);
+    }
+
+    /// <summary>
+    /// Stop monitoring and dispose the timer.
+    /// </summary>
+    public void Stop()
+    {
+        _timer?.Dispose();
+        _timer = null;
+    }
+
+    public void Dispose()
+    {
+        Stop();
+    }
+
+    private void OnTimerElapsed(object? state)
+    {
+        if (_staleFired) return;
+        _staleFired = true;
+        Stale?.Invoke();
+    }
+}