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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbHistorianDataSource.cs

@@ -221,7 +221,7 @@ public sealed class QuestDbHistorianDataSource : IHistorianDataSource
                     values.Add(new HistoryModifiedValue(prior, modifiedAt,
                         (HistoryModificationType)QuestDbConnection.ReadKind(reader, 9),
                         reader.IsDBNull(10) ? string.Empty : reader.GetString(10),
-                        reader.GetInt64(7)));
+                        reader.GetInt64(7), sourceTime));
                 }
             }