fix(historian): #687 Gateway raw reads drop AVEVA's synthetic start-boundary row

An AVEVA Full raw read leads with a row stamped at the query start (the value in
effect, Quality=133, or a no-data NULL row); it was returned as a stored sample.
Raw reads now use the #681 classification (RawAggregateReader.RequestStartFor /
IsWindowSample — one authority, by timestamp, since a stored NULL looks the same
on the wire): request from 1 ms early with NumValuesPerNode + 16 rows, keep rows
at or after StartTime, drain the whole stream. A full page still holds exactly
the cap, so #669 paging/resume/tie-cluster reads are unchanged; a margin that
swallows the allowance fails the node rather than ending paging early.
ReturnBounds is not plumbed on any backend (documented).

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

@@ -74,6 +74,31 @@ internal static class RawAggregateReader
     /// <summary>Rows the earlier-data probe asks for (a possible boundary row plus the first stored sample).</summary>
     internal const int ProbeRows = 2;
 
+    /// <summary>
+    ///     The <c>ReadRaw</c> start for a window that begins at <paramref name="windowStartUtc"/>: <see cref="BoundaryMargin"/>
+    ///     earlier, so the AVEVA start-boundary row — stamped exactly at the QUERY start, carrying the value in effect there
+    ///     (<c>Quality = 133</c>) or, with no earlier data, a NULL row (value 0, Bad OPC quality) — lands BEFORE the window
+    ///     (<see cref="IsWindowSample"/>). The one place both the raw-evaluated aggregates (#681) and a raw HistoryRead
+    ///     (#687) decide what the request start is. No margin when the window starts within one margin of
+    ///     <see cref="DateTime.MinValue"/>.
+    /// </summary>
+    /// <param name="windowStartUtc">The window's (inclusive) start.</param>
+    /// <returns>The start to request.</returns>
+    public static DateTime RequestStartFor(DateTime windowStartUtc) =>
+        windowStartUtc.Ticks > BoundaryMargin.Ticks ? windowStartUtc - BoundaryMargin : windowStartUtc;
+
+    /// <summary>
+    ///     True when a row of a stream requested from <see cref="RequestStartFor"/> is a stored sample of the window that
+    ///     begins at <paramref name="windowStartUtc"/>: it is stamped at or after the window start. A row before it is the
+    ///     AVEVA boundary row (whatever its value or quality — a stored NULL looks the same on the wire, so the timestamp is
+    ///     the evidence) or a stored sample inside the margin; a row without a timestamp is never a sample.
+    /// </summary>
+    /// <param name="raw">The gateway row.</param>
+    /// <param name="windowStartUtc">The window's (inclusive) start.</param>
+    /// <returns>Whether the row belongs to the window.</returns>
+    public static bool IsWindowSample(HistorianSample raw, DateTime windowStartUtc) =>
+        raw.Timestamp is { } ts && ts.ToDateTime() >= windowStartUtc;
+
     /// <summary>True for the aggregates this reader serves: the per-sample families of <see cref="AggregateFamilies"/>.</summary>
     /// <param name="aggregate">Requested aggregate.</param>
     /// <returns>Whether the aggregate is evaluated from raw samples.</returns>
@@ -113,7 +138,7 @@ internal static class RawAggregateReader
     {
         if (plan.BucketCount == 0) return [];
         var needsRange = AggregateEvaluator.NeedsDataRange(aggregate, plan);
-        var requestStart = plan.StartUtc.Ticks > BoundaryMargin.Ticks ? plan.StartUtc - BoundaryMargin : plan.StartUtc;
+        var requestStart = RequestStartFor(plan.StartUtc);
         var maxValues = MaxValuesFor(maxWindowRows);
 
         var buckets = new RawBucketAccumulator(aggregate == HistoryAggregateType.MaximumActualTime);