feat(health): additive script-scheduler stats on ISiteHealthCollector + SiteHealthReport (S2/UA5)

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Health/SiteHealthReport.cs

@@ -54,7 +54,26 @@ public record SiteHealthReport(
     // signal surfaces a forgotten site's stale backlog (count alone hides age).
     // Additive-only trailing member — defaults to null for producers / tests that
     // don't populate it.
-    double? OldestParkedMessageAgeSeconds = null);
+    double? OldestParkedMessageAgeSeconds = null)
+{
+    // Script-execution-scheduler gauges (S2/UA5), surfaced additively as init
+    // properties (rather than positional parameters) so the constructor signature
+    // is untouched. Point-in-time, refreshed on the site by ScriptSchedulerStatsReporter;
+    // safe defaults for producers / tests that don't populate them.
+
+    /// <summary>Script tasks waiting to run on the dedicated script-execution scheduler.</summary>
+    public int ScriptQueueDepth { get; init; }
+
+    /// <summary>Script-execution worker threads currently running a script body.</summary>
+    public int ScriptBusyThreads { get; init; }
+
+    /// <summary>
+    /// Age (seconds) of the oldest in-flight script across the scheduler's worker
+    /// threads, or null when the pool is idle. A large value with all threads busy
+    /// indicates a stuck/blocked script holding a dedicated thread.
+    /// </summary>
+    public double? ScriptOldestBusyAgeSeconds { get; init; }
+}
 
 /// <summary>
 /// Broadcast wrapper used between central nodes to keep per-node