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/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/IHistorianDataSource.cs

@@ -120,9 +120,9 @@ public interface IHistorianDataSource : IDisposable
     /// <summary>
     ///     Whether this backend can serve a reverse (newest-first) events read — the reverse overload of
     ///     <see cref="ReadEventsAsync(string?, DateTime, DateTime, int, bool, CancellationToken)"/> (Gitea #675). The server
-    ///     checks it BEFORE calling the source: an explicit reverse read (<c>StartTime &gt; EndTime</c>) on a backend that
-    ///     cannot is answered <c>BadHistoryOperationUnsupported</c> without a call, and an end-only capped read, which Part 11
-    ///     reads backward, keeps its forward <c>[MinValue, EndTime)</c> meaning there. Default <c>false</c>; an
+    ///     checks it BEFORE calling the source: a reverse read (explicit <c>StartTime &gt; EndTime</c>, or an end-only capped
+    ///     read, which Part 11 reads backward) on a backend that cannot is never sent the reverse overload — since Gitea
+    ///     #691.4 the server walks it back from bounded FORWARD reads instead (#675 refused it). Default <c>false</c>; an
     ///     implementation (or a wrapper) declares it itself — a guard test pins that.
     /// </summary>
     bool SupportsReverseEvents => false;
@@ -144,9 +144,9 @@ public interface IHistorianDataSource : IDisposable
     ///     <para>
     ///         The default forwards a forward read to the five-argument overload and throws
     ///         <see cref="NotSupportedException"/> for a reverse one — a backend that cannot read newest-first (Gateway,
-    ///         Null). The server never relies on that throw: it consults <see cref="SupportsReverseEvents"/> first and
-    ///         answers <c>BadHistoryOperationUnsupported</c> without calling, rather than returning the earliest events in
-    ///         ascending order as if they were the answer. A backend that serves reverse must override BOTH members.
+    ///         Null). The server never relies on that throw: it consults <see cref="SupportsReverseEvents"/> first and walks
+    ///         such a read back from forward reads (#691.4), never returning the earliest events in ascending order as if
+    ///         they were the answer. A backend that serves reverse natively must override BOTH members.
     ///     </para>
     /// </remarks>
     /// <param name="sourceName">The source name to filter events, or null to return events from all sources.</param>

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

@@ -61,8 +61,8 @@ public sealed class NullHistorianDataSource : IHistorianDataSource
     public bool SupportsReverseRaw => true;
 
     /// <inheritdoc />
-    /// <remarks><c>false</c> (Gitea #675): there is no history to read in either direction, and an explicit reverse read
-    /// is answered <c>BadHistoryOperationUnsupported</c> like any backend that cannot read newest-first.</remarks>
+    /// <remarks><c>false</c> (Gitea #675): there is no history to read in either direction; the server walks a reverse
+    /// read back from (empty) forward reads like any backend that cannot read newest-first (#691.4).</remarks>
     public bool SupportsReverseEvents => false;
 
     /// <inheritdoc />