feat(health): oldest-parked-message age on the site health report (parked-retention aging signal)

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/SiteHealthCollector.cs

@@ -25,6 +25,10 @@ public class SiteHealthCollector : ISiteHealthCollector
     private IReadOnlyDictionary<string, int> _sfBufferDepths = new Dictionary<string, int>();
     private int _deployedInstanceCount, _enabledInstanceCount, _disabledInstanceCount;
     private int _parkedMessageCount;
+    // Age (seconds) of the oldest parked message, stored as raw double bits so it can
+    // be read/written atomically via Interlocked (double? is neither atomic nor
+    // volatile-able). NaN is the "no parked rows" sentinel (ages are always >= 0).
+    private long _oldestParkedAgeBits = BitConverter.DoubleToInt64Bits(double.NaN);
     private volatile string _nodeHostname = "";
     private volatile IReadOnlyList<Commons.Messages.Health.NodeStatus>? _clusterNodes;
     private volatile bool _isActiveNode;
@@ -123,6 +127,20 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Exchange(ref _parkedMessageCount, count);
     }
 
+    /// <inheritdoc />
+    public void SetOldestParkedMessageAgeSeconds(double? ageSeconds)
+    {
+        Interlocked.Exchange(ref _oldestParkedAgeBits,
+            BitConverter.DoubleToInt64Bits(ageSeconds ?? double.NaN));
+    }
+
+    /// <summary>Reads the atomically-stored oldest-parked age, mapping the NaN sentinel back to null.</summary>
+    private double? ReadOldestParkedAgeSeconds()
+    {
+        var value = BitConverter.Int64BitsToDouble(Interlocked.Read(ref _oldestParkedAgeBits));
+        return double.IsNaN(value) ? null : value;
+    }
+
     /// <inheritdoc />
     public void SetNodeHostname(string hostname) => _nodeHostname = hostname;
 
@@ -214,6 +232,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             SiteAuditWriteFailures: siteAuditWriteFailures,
             AuditRedactionFailure: auditRedactionFailures,
             SiteAuditBacklog: _siteAuditBacklog,
-            SiteEventLogWriteFailures: Interlocked.Read(ref _siteEventLogWriteFailures));
+            SiteEventLogWriteFailures: Interlocked.Read(ref _siteEventLogWriteFailures),
+            OldestParkedMessageAgeSeconds: ReadOldestParkedAgeSeconds());
     }
 }