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).

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/GatewayHistorianDataSourceTests.cs

@@ -106,7 +106,7 @@ public sealed class GatewayHistorianDataSourceTests
         var t0 = new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
         var t1 = t0.AddSeconds(1);
         fake.AtTimeSamples = new[] { new HistorianSample { NumericValue = 5.0, OpcQuality = 192, Timestamp = Timestamp.FromDateTime(t0) } };
-        var ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance);
+        var ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance, atTimeFromRaw: false); // #693 kill switch
         var r = await ds.ReadAtTimeAsync("T", new[] { t0, t1 }, TestContext.Current.CancellationToken);
         Assert.Equal(2, r.Samples.Count);          // exactly one per requested ts, in order
         Assert.Equal(5.0, r.Samples[0].Value);
@@ -122,7 +122,7 @@ public sealed class GatewayHistorianDataSourceTests
         {
             new HistorianSample { NumericValue = 0, OpcQuality = 0, Quality = 1, Timestamp = Timestamp.FromDateTime(t0) },
         };
-        var ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance);
+        var ds = new GatewayHistorianDataSource(fake, NullLogger<GatewayHistorianDataSource>.Instance, atTimeFromRaw: false); // #693 kill switch
         var r = await ds.ReadAtTimeAsync("T", new[] { t0 }, TestContext.Current.CancellationToken);
         Assert.Null(r.Samples[0].Value);
         Assert.Equal(0x80000000u, r.Samples[0].StatusCode);