fix(historian): #686 review — bounded, density-aware reverse raw walk; uncapped Gateway raw reads

1. Uncapped reverse raw read on a forward-only backend = ONE forward read of the
   window, filtered and reversed in memory (was a segment-by-segment walk).
2. Segment width is aimed at ~half the row budget from the density just observed
   (overflow: from the truncated read's own sample span) and carried page to page
   in HistoryContinuationState.SegmentWidthTicks.
3. The walk starts no later than now + 1 day and never reads before the epoch.
4. Request bounds are rounded outward to the millisecond, so a row AVEVA
   fabricates at a rounded-up query start cannot land inside the segment.
5. Budget M = max(1, MaxTieClusterOverfetch); reads ask M + 2 rows, completeness is
   'short OR last row past the segment', and a read crowded by rows before the
   segment is re-read once (<= 2M + 4). A cluster of exactly M, neighbouring
   clusters each within M, and M <= 1 are all served; only one timestamp over M
   is refused. ServerHistorianOptions text updated.
6. The raw and events tie-path oversized-cluster refusal is BadTooManyOperations
   (was BadHistoryOperationUnsupported), consistent with the walk.
7. GatewayHistorianDataSource.ReadRawAsync: a caller cancellation is not a backend
   failure; NumValuesPerNode = 0 is sent as int.MaxValue (the client package and
   the gateway reject max_values <= 0, so every uncapped Gateway raw read failed);
   a MaxValuesCeiling refusal is a HistoryReadLimitExceededException.
8. Tests: upper-edge filter, epoch floor, look-ahead, ms alignment, exact/adjacent
   clusters, minimal budget, carried density, and the walk over the real Gateway
   source + FakeHistorianGatewayClient (OpcUaServer internals visible to it).

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/HistoryPaging.cs

@@ -51,6 +51,10 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
 ///     <c>(EndUtc, NextStartUtc]</c>. A resumed events read takes its direction from here, never from the resume
 ///     request's details. Raw keeps deriving it from the window (<c>NextStartUtc &gt; EndUtc</c>).
 /// </param>
+/// <param name="SegmentWidthTicks">
+///     <see cref="HistoryCursorKind.Raw"/> reverse reads on a forward-only backend only (Gitea #686): the segment width the
+///     walk-back ended on, so the next page starts at the observed density instead of re-discovering it. 0 = none.
+/// </param>
 internal sealed record HistoryContinuationState(
     string Tagname,
     DateTime NextStartUtc,
@@ -60,7 +64,8 @@ internal sealed record HistoryContinuationState(
     HistoryCursorKind Kind = HistoryCursorKind.Raw,
     long LastModifiedId = 0,
     DateTime WindowStartUtc = default,
-    bool Reverse = false);
+    bool Reverse = false,
+    long SegmentWidthTicks = 0);
 
 /// <summary>The HistoryRead kind that issued a <see cref="HistoryContinuationState"/>.</summary>
 internal enum HistoryCursorKind