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.

tests/ScadaLink.Commons.Tests/Types/StaleTagMonitorTests.cs

@@ -0,0 +1,129 @@
+using ScadaLink.Commons.Types;
+
+namespace ScadaLink.Commons.Tests.Types;
+
+public class StaleTagMonitorTests
+{
+    [Fact]
+    public void Constructor_ZeroTimeSpan_Throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => new StaleTagMonitor(TimeSpan.Zero));
+    }
+
+    [Fact]
+    public void Constructor_NegativeTimeSpan_Throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => new StaleTagMonitor(TimeSpan.FromSeconds(-1)));
+    }
+
+    [Fact]
+    public async Task Stale_FiresAfterMaxSilence()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(100));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+
+        await Task.Delay(300);
+        Assert.Equal(1, staleCount);
+    }
+
+    [Fact]
+    public async Task Stale_FiresOnlyOnce()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(50));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+
+        await Task.Delay(300);
+        Assert.Equal(1, staleCount);
+    }
+
+    [Fact]
+    public async Task OnValueReceived_ResetsTimer()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(200));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+
+        // Keep resetting before the 200ms deadline
+        for (int i = 0; i < 5; i++)
+        {
+            await Task.Delay(100);
+            monitor.OnValueReceived();
+        }
+
+        // Should not have gone stale
+        Assert.Equal(0, staleCount);
+    }
+
+    [Fact]
+    public async Task OnValueReceived_AllowsStaleAfterSilence()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(100));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+
+        // Reset once
+        await Task.Delay(50);
+        monitor.OnValueReceived();
+
+        // Then go silent
+        await Task.Delay(250);
+        Assert.Equal(1, staleCount);
+    }
+
+    [Fact]
+    public async Task OnValueReceived_ResetsStaleFlag_AllowsSecondFire()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(100));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+
+        // Wait for first stale
+        await Task.Delay(250);
+        Assert.Equal(1, staleCount);
+
+        // Reset — should allow second stale fire
+        monitor.OnValueReceived();
+        await Task.Delay(250);
+        Assert.Equal(2, staleCount);
+    }
+
+    [Fact]
+    public async Task Stop_PreventsStale()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(50));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+        monitor.Stop();
+
+        await Task.Delay(200);
+        Assert.Equal(0, staleCount);
+    }
+
+    [Fact]
+    public async Task Dispose_PreventsStale()
+    {
+        var monitor = new StaleTagMonitor(TimeSpan.FromMilliseconds(50));
+        var staleCount = 0;
+        monitor.Stale += () => Interlocked.Increment(ref staleCount);
+        monitor.Start();
+        monitor.Dispose();
+
+        await Task.Delay(200);
+        Assert.Equal(0, staleCount);
+    }
+
+    [Fact]
+    public void MaxSilence_ReturnsConfiguredValue()
+    {
+        using var monitor = new StaleTagMonitor(TimeSpan.FromSeconds(42));
+        Assert.Equal(TimeSpan.FromSeconds(42), monitor.MaxSilence);
+    }
+}