fix(historian): #693 Gateway AtTime evaluated from raw reads with the shared AtTimeEvaluator

Gateway HistoryRead-AtTime passed AVEVA's own interpolation through: live it
answered the previous value as Good at a stored NULL's own time, null + Bad at
a Good sample right after a NULL, a drifted 99.99999999944 for a stored 100,
and no Interpolated bit. It is now served from raw gateway reads and evaluated
with Sql.Common's AtTimeEvaluator, as the SQL backends do (Part 11 §6.5.4).

GatewayAtTimeReader: one <=1,024-row forward window read [first-1ms, last+1ms]
for the batch, a walk-back (density-seeded, x4 growth, whole-remainder shortcut,
bisects dense history) for the first time's prev, and an 18-row probe for the
last time's next; truncated window reads continue or jump with a density-sized
margin. All bounds whole ms; a row counts only strictly inside its request
(AVEVA's boundary row never a sample); Bad rows value-less (#690); never past
now + 1 day; strict-gateway ceiling learned (#691.3) without re-probing.
10 dense times = 1 read (exact) / 3 (interpolated); 10 hourly = 12.

Kill switch ServerHistorian:GatewayAtTimeFromRaw (default true; false = the
old AVEVA passthrough). Known limit: the wire carries no data type, so numeric
tags interpolate as Double (Boolean/integer nodes are not stepped/rounded as on
the SQL backends).

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

@@ -46,6 +46,10 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     private readonly ILogger<GatewayHistorianDataSource> _logger;
     private readonly int _maxRawRows;
     private readonly TimeProvider _time;
+    private readonly bool _atTimeFromRaw;
+
+    /// <summary>#693: AtTime from raw reads (true) or AVEVA's own interpolation (the kill switch).</summary>
+    internal bool AtTimeFromRaw => _atTimeFromRaw;
 
     /// <summary>A strict gateway's row ceiling, learned from its first refusal (#691.3).</summary>
     private readonly GatewayRowCeiling _ceiling = new();
@@ -67,9 +71,15 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     ///     &lt;= 0 = unbounded). Past it the node fails <c>BadTooManyOperations</c>.
     /// </param>
     /// <param name="timeProvider">Clock for result server timestamps and the raw look-ahead horizon; null = system.</param>
+    /// <param name="atTimeFromRaw">
+    ///     #693: serve AtTime from raw reads with the shared AtTime rules (<see cref="GatewayAtTimeReader"/>; default);
+    ///     <c>false</c> = AVEVA's own interpolation passed through (<c>ServerHistorian:GatewayAtTimeFromRaw</c> kill switch).
+    /// </param>
     public GatewayHistorianDataSource(IHistorianGatewayClient client, ILogger<GatewayHistorianDataSource> logger,
-        int maxRawRows = TimeWeightedAggregator.DefaultMaxWindowRows, TimeProvider? timeProvider = null)
+        int maxRawRows = TimeWeightedAggregator.DefaultMaxWindowRows, TimeProvider? timeProvider = null,
+        bool atTimeFromRaw = true)
     {
+        _atTimeFromRaw = atTimeFromRaw;
         ArgumentNullException.ThrowIfNull(client);
         ArgumentNullException.ThrowIfNull(logger);
         _client = client;
@@ -364,8 +374,48 @@ public sealed class GatewayHistorianDataSource : IHistorianDataSource, IAsyncDis
     internal static bool IsMaxValuesCeilingRejection(Exception ex) => GatewayReadLimits.IsCeilingRejection(ex);
 
     /// <inheritdoc />
+    /// <remarks>
+    ///     #693: served from RAW gateway reads — each requested time's stored sample at-or-before and after it — evaluated with
+    ///     the SQL backends' shared <see cref="AtTimeEvaluator"/> (Part 11 §6.5.4), so the Gateway answers what SqlServer and
+    ///     QuestDB answer for the same stored samples; see <see cref="GatewayAtTimeReader"/> for the read plan and its cost.
+    ///     AVEVA's own interpolated answer (the gateway's <c>ReadAtTime</c>) returned the previous value as Good at a stored
+    ///     NULL's time, null + Bad at a Good sample right after a NULL, a drifted value at an exact stored sample, and no
+    ///     Interpolated bit; it is only used when <c>ServerHistorian:GatewayAtTimeFromRaw = false</c> (kill switch).
+    ///     A gateway value is a Double (or a String) whatever the node's type, so every numeric tag is interpolated linearly
+    ///     (on the SQL backends a Boolean is stepped and an Int64 rounded — the gateway wire carries no type to tell).
+    /// </remarks>
     public async Task<HistoryReadResult> ReadAtTimeAsync(
         string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(timestampsUtc);
+        if (!_atTimeFromRaw) return await ReadAtTimeNativeAsync(fullReference, timestampsUtc, cancellationToken).ConfigureAwait(false);
+        try
+        {
+            var now = _time.GetUtcNow().UtcDateTime;
+            var values = await GatewayAtTimeReader
+                .ReadAsync(_client, _ceiling, fullReference, timestampsUtc, now, cancellationToken)
+                .ConfigureAwait(false);
+            RecordOutcome(success: true, error: null);
+            return new HistoryReadResult(values, ContinuationPoint: null);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw; // caller-requested, not a backend fault
+        }
+        catch (HistoryReadLimitExceededException)
+        {
+            throw; // a client-request limit (a pathological tie cluster), not a backend fault
+        }
+        catch (Exception ex)
+        {
+            RecordReadFailure(ex);
+            throw;
+        }
+    }
+
+    /// <summary>The pre-#693 AtTime: AVEVA's own interpolation, aligned to the request (kill switch only).</summary>
+    private async Task<HistoryReadResult> ReadAtTimeNativeAsync(
+        string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken)
     {
         try
         {