fix(health): flag metrics-stale for never-reported sites via FirstSeenAt anchor — null LastReportReceivedAt no longer skips the check (plan R2-01 T8)

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

@@ -47,6 +47,7 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
                     LatestReport = report,
                     LastReportReceivedAt = now,
                     LastHeartbeatAt = now,
+                    FirstSeenAt = now,
                     LastSequenceNumber = report.SequenceNumber,
                     IsOnline = true,
                     IsMetricsStale = false
@@ -117,6 +118,7 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
                     LatestReport = null,
                     LastReportReceivedAt = null,
                     LastHeartbeatAt = receivedAt,
+                    FirstSeenAt = receivedAt,
                     LastSequenceNumber = 0,
                     IsOnline = true
                 };
@@ -297,16 +299,22 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
             // when no report has arrived within MetricsStaleTimeout. Distinct from
             // offline — a site whose HealthReportSender crashed keeps heartbeating
             // and would otherwise show "online with frozen metrics forever".
-            if (state.LastReportReceivedAt is { } lastReport
-                && now - lastReport > _options.MetricsStaleTimeout
+            // A site that has NEVER reported (heartbeat-only registration,
+            // LastReportReceivedAt == null) anchors on FirstSeenAt instead —
+            // otherwise a pipeline dead from first boot is never flagged (N3).
+            var reportAnchor = state.LastReportReceivedAt ?? state.FirstSeenAt;
+            if (reportAnchor is { } anchor
+                && now - anchor > _options.MetricsStaleTimeout
                 && !state.IsMetricsStale)
             {
                 var stale = state with { IsMetricsStale = true };
                 if (_siteStates.TryUpdate(kvp.Key, stale, state))
                 {
                     _logger.LogWarning(
-                        "Site {SiteId} metrics are stale — online (heartbeats) but no report for {Elapsed}s (timeout: {Timeout}s)",
-                        state.SiteId, (now - lastReport).TotalSeconds, _options.MetricsStaleTimeout.TotalSeconds);
+                        "Site {SiteId} metrics are stale — online (heartbeats) but no report for {Elapsed}s{NeverNote} (timeout: {Timeout}s)",
+                        state.SiteId, (now - anchor).TotalSeconds,
+                        state.LastReportReceivedAt is null ? " (never reported since first contact)" : string.Empty,
+                        _options.MetricsStaleTimeout.TotalSeconds);
                 }
                 // CAS loss ⇒ a fresh report/heartbeat swapped in ⇒ correct to skip.
             }

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

@@ -40,6 +40,17 @@ public sealed record SiteHealthState
     /// </summary>
     public DateTimeOffset LastHeartbeatAt { get; init; }
 
+    /// <summary>
+    /// Gets the instant this site was first observed by the aggregator (via a
+    /// report or a heartbeat); <c>null</c> only for states not built by the
+    /// aggregator (hand-constructed fixtures). Anchors metrics-staleness for a
+    /// site that has NEVER delivered a report (review 01 round-2 N3): with
+    /// <see cref="LastReportReceivedAt"/> null the staleness check previously
+    /// skipped the site forever. <see cref="LastHeartbeatAt"/> cannot anchor
+    /// this — every heartbeat advances it.
+    /// </summary>
+    public DateTimeOffset? FirstSeenAt { get; init; }
+
     /// <summary>Gets the sequence number of the last accepted health report, used to reject out-of-order duplicates.</summary>
     public long LastSequenceNumber { get; init; }
     /// <summary>Gets a value indicating whether the site is currently considered online.</summary>