fix(historian): #691.4 walk reverse events reads back on forward-only backends (Gateway)

A reverse or end-only capped events read on a backend without
SupportsReverseEvents (Gateway, Null) was refused
BadHistoryOperationUnsupported (explicit reverse) or answered with the
OLDEST N ascending (end-only, #675). It is now served newest-first by
the same ReverseWalk as raw reads: page-sized ms-aligned segments,
never answered from a read the source flagged truncated, ties sorted by
EventId (the gateway orders by event time only - the fake shuffles
them per call), one extra event walked to signal 'more', uncapped = one
forward read reversed, and the width + proven-empty stretch carried in
the events cursor. The oversized-cluster over-fetch keeps exactly the
events at T in the same order. A tie cluster over the budget is
BadTooManyOperations. Docs note the gateway's EventReadMaxRows must be
unlimited (it truncates silently).

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

@@ -393,8 +393,9 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
 
     /// <inheritdoc />
     /// <remarks>
-    ///     <c>false</c>: the gateway's <c>ReadEvents</c> streams ascending only (Gitea #675). The server therefore refuses an
-    ///     explicit reverse events read without calling, and keeps an end-only capped read forward.
+    ///     <c>false</c>: the gateway's <c>ReadEvents</c> streams ascending only (Gitea #675). The server never sends it a
+    ///     reverse read; since #691.4 it walks one back from bounded forward reads (ties sorted by EventId — the gateway
+    ///     orders by event time only).
     /// </remarks>
     public bool SupportsReverseEvents => false;
 
@@ -404,10 +405,9 @@ 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.
+    ///     A reverse (newest-first) read is refused with <see cref="NotSupportedException"/> (Gitea #675) — the gateway's
+    ///     <c>ReadEvents</c> streams ascending only and rejects <c>end &lt; start</c>. The server never calls this with
+    ///     <c>reverse</c>: it walks a reverse read back from bounded forward reads (#691.4). Nothing is sent.
     /// </remarks>
     public Task<HistoricalEventsResult> ReadEventsAsync(
         string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, bool reverse,