feat(historian): #675 reverse HistoryRead-Events paging + end-only backward reads

- IHistorianDataSource.ReadEventsAsync(..., bool reverse, ...) overload: reverse = Part 11 window (lo, hi]
  newest-first by (time, id) DESC. Default forwards forward reads and throws NotSupportedException for
  reverse (Null); Gateway refuses explicitly (ascending-only stream). Server answers
  BadHistoryOperationUnsupported instead of silently returning the earliest N ascending.
- SqlServer / QuestDB serve reverse natively (ORDER BY time DESC, id DESC + TOP/LIMIT).
- Node manager: reverse event reads page with continuation points; direction stored in the cursor
  (HistoryContinuationState.Reverse) so a resume continues in the cursor's direction; reverse tie clusters
  over-fetched forward and sliced from the newest end (SliceTieCluster(reverse:)); end-only reads with a
  cap page backward from EndTime (mirrors raw).
- Resumed event pages over-fetch by the boundary skip, so every non-final page is a full cap page.
- Tests: node-manager reverse paging (ties, clusters, coarse resolution, overfetch bound, resume
  direction both ways, end-only, unsupported backend); shared EventReadConformance on SqlServer +
  QuestDB; over-the-wire reverse paging; offline Gateway pin of the UTC DateTime.MaxValue bound through
  the published client's proto conversion (in-process gRPC capture) and FILETIME.
- docs/Historian.md event paging section.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayHistorianDataSource.cs

@@ -125,6 +125,21 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
         }
     }
 
+    /// <inheritdoc />
+    /// <remarks>
+    ///     A reverse (newest-first) read is refused with <see cref="NotSupportedException"/> — the server reports
+    ///     <c>BadHistoryOperationUnsupported</c> (Gitea #675). The gateway's <c>ReadEvents</c> streams ascending only and
+    ///     rejects <c>end &lt; start</c>, so the only way to serve "the latest N" would be to drain the whole window and
+    ///     keep its tail, an unbounded read the cap exists to prevent. Nothing is sent.
+    /// </remarks>
+    public Task<HistoricalEventsResult> ReadEventsAsync(
+        string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse,
+        CancellationToken cancellationToken) =>
+        reverse
+            ? throw new NotSupportedException(
+                "The HistorianGateway backend reads events oldest-first only; a reverse (newest-first) events read is not supported.")
+            : ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, cancellationToken);
+
     /// <inheritdoc />
     public async Task<HistoricalEventsResult> ReadEventsAsync(
         string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents,