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.

src/Server/ZB.MOM.WW.OtOpcUa.Host/appsettings.json

@@ -35,7 +35,9 @@
     "AllowUntrustedServerCertificate": false,
     "CaCertificatePath": null,
     "CallTimeout": "00:00:30",
-    "MaxTieClusterOverfetch": 65536
+    "MaxTieClusterOverfetch": 65536,
+    "EventReadHorizon": "00:00:00",
+    "_EventReadHorizonComment": "Gitea #694, opt-in. 00:00:00 = no horizon: HistoryReadEvents is exact to each read's window (on the Gateway a reverse/end-only events read proves 'nothing older' with one read down to 1970 - seconds on a large Runtime.dbo.Events). A positive span (e.g. 365.00:00:00) never reads events older than now minus it, in either direction - LOSSY: older events become invisible to HistoryReadEvents."
   },
   "ContinuousHistorization": {
     "_comment": "Continuous historization of driver (non-Galaxy) tag values: a crash-safe FasterLog outbox + recorder draining to the ServerHistorian gateway's WriteLiveValues. Disabled => no recorder is spawned. Requires ServerHistorian to be configured; the gateway connection (endpoint/key/TLS) is sourced from the ServerHistorian section, not here.",