fix(historian): #694 page raw reads at a strict gateway's learned ceiling; opt-in EventReadHorizon

Item 1: IHistorianDataSource.MaxRawValuesPerRead (Gateway: learned ceiling - 16 = L; 0 elsewhere, guard-tested).
RawPageLimit plans every raw page on top of #692's lookahead: with L > 0 the read (page P + boundary ties k
+ lookahead) is <= L, P = min(N, L - k - 1) (uncapped: L - k - 1); no room => straight to the tie-cluster
over-fetch (never an uncapped or underflowing request). One re-plan + re-read when the read taught a new
limit or was refused for crowding pre-window rows (HistoryReadLimitExceededException.PreWindowRows): the
page shrinks by them. Over-fetch bound min(MaxTieClusterOverfetch, L - 1); slice size never 0.
Review fixes: F1 a window starting at/before the epoch counts AVEVA's synthetic row as pre-window (a
ceiling-truncated read was reported complete); F2 the page path disables ReverseRawReader's own re-run
(cold end-only ceiling 20/N=3000 cost 3,023 reads, now <= 40); F3 crowded look-back re-read around, not
refused; F4 the #687 crowding re-read room at least doubles (up to 8 re-reads), so a cluster of hundreds
just before a resume point no longer fails on a default gateway; a crowding refusal that leaves no room
is BadTooManyOperations, never a tick-by-tick cursor.

Item 2: ServerHistorian:EventReadHorizon (default 00:00:00 = exact, unchanged). When set, every fresh
HistoryReadEvents window's earlier bound is raised to now - horizon (both directions, all backends).
Lossy by design; documented.

Item 3 (native TimeAverage/Total re-check) stays blocked on historiangw#27.

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/OpcUa/OtOpcUaServerHostedServiceHistoryUpdateTests.cs

@@ -84,6 +84,18 @@ public sealed class OtOpcUaServerHostedServiceHistoryUpdateTests
         Service(view: null, config: config).HonorReturnBounds.ShouldBeFalse();
     }
 
+    /// <summary>#694: the events horizon is bound from <c>ServerHistorian:EventReadHorizon</c>, default none.</summary>
+    [Fact]
+    public void The_event_read_horizon_comes_from_options_and_defaults_off()
+    {
+        Service(view: null).EventReadHorizon.ShouldBe(TimeSpan.Zero);
+        var config = new ConfigurationBuilder().AddInMemoryCollection(new Dictionary<string, string?>
+        {
+            ["ServerHistorian:EventReadHorizon"] = "365.00:00:00",
+        }).Build();
+        Service(view: null, config: config).EventReadHorizon.ShouldBe(TimeSpan.FromDays(365));
+    }
+
     private static OtOpcUaServerHostedService Service(
         IHistoryUpdateRoleView? view, IDriverPrimaryRoleSource? primary = null, IConfiguration? config = null,
         TimeProvider? clock = null) => new(