fix(historian): #668 review follow-ups (explicit modified source time, paging tests, ACL test, QuestDB orphan warning, docs)

- HistoryModifiedValue gains explicit SourceTimeUtc (SqlServer + QuestDB populate it); the node manager's resume cursor uses it instead of the prior value's nullable source timestamp (MinValue for inserts).
- ACL test now uses a trail-returning data source and asserts Good + data on the allow leg.
- Node-manager tests: end-only backward open-ended paging, reverse resume cursor, null-prior cursor.
- QuestDB updater logs a Warning (tag + time range) when a sample write fails after its trail rows landed.
- docs/Historian.md: rolling-upgrade window, IDENTITY-vs-commit ordering, QuestDB cross-node ModifiedId clock ordering.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/HistoryModifiedContracts.cs

@@ -51,12 +51,17 @@ public enum HistoryModificationType : byte
 /// <param name="UpdateType">The kind of modification.</param>
 /// <param name="UserName">The principal that made it.</param>
 /// <param name="ModifiedId">The backend's ordering key within one source timestamp (paging cursor).</param>
+/// <param name="SourceTimeUtc">
+///     The modified sample's source time as stored on the trail row — the paging cursor's time component. Explicit
+///     because an insert's <see cref="Value"/> has no prior value and so cannot be relied on to carry it.
+/// </param>
 public sealed record HistoryModifiedValue(
     DataValueSnapshot Value,
     DateTime ModificationTimeUtc,
     HistoryModificationType UpdateType,
     string UserName,
-    long ModifiedId);
+    long ModifiedId,
+    DateTime SourceTimeUtc);
 
 /// <summary>Resume position for a paged modified-value read: strictly after (or before, reversed) this key.</summary>
 /// <param name="SourceTimeUtc">The last returned modification's source time.</param>