refactor(kpi): K2/K6/K7 review fixups — empty-batch guard + sealed repo + uniform TryAddEnumerable + KPI-age doc fidelity + coverage

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/Kpi/NotificationOutboxKpiSampleSource.cs

@@ -20,7 +20,9 @@ namespace ZB.MOM.WW.ScadaBridge.NotificationOutbox.Kpi;
 /// <c>capturedAtUtc</c> rather than wall-clock <c>now</c>: the stuck cutoff is
 /// <c>capturedAtUtc - <see cref="NotificationOutboxOptions.StuckAgeThreshold"/></c> and the
 /// delivered window is <c>capturedAtUtc - <see cref="NotificationOutboxOptions.DeliveredKpiWindow"/></c>.
-/// So a sample captured at the same instant equals the live tile.
+/// The COUNT metrics (queueDepth, stuckCount, parkedCount, deliveredLastInterval) equal the
+/// live tile at the same instant; the <c>oldestPendingAgeSeconds</c> metric is computed against
+/// the repository's internal clock and may differ from the live tile by the query-execution latency.
 /// </para>
 /// <para>
 /// Emits Global (<c>ScopeKey == null</c>), per-Site (<c>ScopeKey == SourceSiteId</c>), and