fix(historian): #693 re-review — AtTime read cap 64 + 16 per time (bursty tags need ~6)

A time inside a gap of a bursty tag costs a 64-row jump read landing in the next
burst plus a bisection back across the gap (~6 reads), so the 64 + 4/time cap
refused a 1 h/day 1 Hz tag asked at noon on 30/100/300 consecutive days
(186/606/1,806 reads) that AVEVA's own AtTime served. The cap is a runaway guard
(HistoryReadDeadline bounds wall clock): now 64 + 16 per distinct time.
Regression tests for x30/x100/x300 (each <= 7 reads a time, values checked).
Docs: DateTime-hinted nodes are stepped but keep the gateway's Double (the
Gateway never historizes DateTime nodes; the SQL backends return DateTime).

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

@@ -41,7 +41,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway;
 ///         2 ms the segment is the whole stretch from the known row (never a 100 ns step).
 ///     </para>
 ///     <para>
-///         <b>Bounded.</b> One request may take at most <c>64 + 4 × distinct times</c> raw reads (<see cref="BaseReads"/>,
+///         <b>Bounded.</b> One request may take at most <c>64 + 16 × distinct times</c> raw reads (<see cref="BaseReads"/>,
 ///         <see cref="ReadsPerTime"/>); past it — or with a strict-gateway ceiling below <see cref="MinBudget"/> rows — the
 ///         node fails <c>BadTooManyOperations</c> with a message naming the limit. A jump read (to a time far past the previous
 ///         read) asks for only <see cref="JumpRows"/> rows, so a thousand times spread over ten years cost about a thousand
@@ -82,8 +82,12 @@ internal static class GatewayAtTimeReader
     /// <summary>Raw reads one request may always take …</summary>
     internal const int BaseReads = 64;
 
-    /// <summary>… plus this many per distinct requested time.</summary>
-    internal const int ReadsPerTime = 4;
+    /// <summary>
+    ///     … plus this many per distinct requested time. A runaway guard, not a budget (<c>HistoryReadDeadline</c> bounds the
+    ///     wall clock): a time inside a gap of a bursty tag legitimately costs a jump read plus a bisection back across the gap
+    ///     (≈ 6 reads — a 1 h/day tag asked at noon on consecutive days), so 4 refused requests AVEVA's own AtTime served.
+    /// </summary>
+    internal const int ReadsPerTime = 16;
 
     /// <summary>The first walk-back segment's width when the window read showed no density.</summary>
     internal static readonly TimeSpan InitialWalkWidth = TimeSpan.FromMinutes(1);