fix(historian): #687 re-review — a row at/before the Unix epoch is never a stored sample

With StartTime at or before the epoch, RequestStartFor has no margin left, so
AVEVA's synthetic no-data/boundary row is stamped exactly at StartTime and was
kept: raw reads returned a fabricated Bad sample at 1970-01-01 and the per-sample
aggregates put it in bucket 0 (Start/End fabricated, Min/Max status Uncertain).
IsWindowSample and the aggregate loop now drop any row with t <= ProbeFloor
(RawAggregateReader.IsAfterFloor), matching the #686 walk-back's assumption.

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

@@ -104,9 +104,18 @@ internal static class RawAggregateReader
     /// <param name="windowEndUtc">The window's end: exclusive, or inclusive when it equals the start.</param>
     /// <returns>Whether the row belongs to the window.</returns>
     public static bool IsWindowSample(HistorianSample raw, DateTime windowStartUtc, DateTime windowEndUtc) =>
-        raw.Timestamp is { } ts && ts.ToDateTime() is var t && t >= windowStartUtc
+        raw.Timestamp is { } ts && ts.ToDateTime() is var t && IsAfterFloor(t) && t >= windowStartUtc
         && (t < windowEndUtc || (t == windowEndUtc && windowStartUtc == windowEndUtc));
 
+    /// <summary>
+    ///     A row stamped at or before <see cref="ProbeFloor"/> (the Unix epoch) is never a stored sample: AVEVA stores nothing
+    ///     there (the server's #686 walk-back assumes the same), and a window starting at or before the epoch gets no 1 ms
+    ///     margin (<see cref="RequestStartFor"/>), so AVEVA's synthetic boundary / no-data row lands exactly at its start.
+    /// </summary>
+    /// <param name="t">The row's timestamp.</param>
+    /// <returns>Whether the row can be a stored sample.</returns>
+    internal static bool IsAfterFloor(DateTime t) => t > ProbeFloor;
+
     /// <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>
@@ -184,6 +193,7 @@ internal static class RawAggregateReader
             if (overCap || raw.Timestamp is null) continue;
             var s = ToStored(raw);
             var t = s.TimeUtc;
+            if (!IsAfterFloor(t)) continue; // the boundary row of a window starting at/before the epoch, never a sample
             if (plan.IndexOf(t) is { } bucket)
             {
                 if (maxWindowRows > 0 && ++windowRows > maxWindowRows)