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.

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/HistoryUpdateOverTheWireTests.cs

@@ -313,7 +313,7 @@ public sealed class HistoryUpdateOverTheWireTests
         private void Record(DateTime t, double? prior, HistoryModificationType type, HistoryUpdateContext who) =>
             _trail.Add(new HistoryModifiedValue(
                 new DataValueSnapshot(prior, prior is null ? StatusCodes.BadNoData : StatusCodes.Good, t, t),
-                DateTime.UtcNow, type, who.Principal, _trail.Count + 1));
+                DateTime.UtcNow, type, who.Principal, _trail.Count + 1, t));
 
         public int UpdateCalls => Volatile.Read(ref _updateCalls);