feat(historian): Part 11 modified-value history on SqlServer + QuestDB (#668)

Every HistoryUpdate a SQL backend applies now leaves a modification trail
(prior value, principal, Part 11 update type, modification time), and
HistoryReadRaw with IsReadModified=true serves it as HistoryModifiedData.

- Core: HistoryUpdateContext threaded into all three IHistorianDataUpdater
  methods; IHistorianDataSource.ReadModifiedAsync (default NotSupported =>
  BadHistoryOperationUnsupported on Gateway/Null); HistoryModified* contracts.
- SqlServer: schema v2 (002_sample_modified.sql, RequiredVersion=2, docs copy);
  trail written by OUTPUT ... INTO on the same MERGE/DELETE statement, so a
  failed update leaves no audit row; retention purges SampleModified per tag on
  the source-time cutoff; ReadModifiedAsync keyed on (SourceTimeUtc, ModifiedId).
- QuestDB: append-only {prefix}sample_modified (TTL like samples), trail row
  written BEFORE the change (orphan-on-crash, never an untracked change);
  existence reads now carry the prior value; ReadModifiedAsync.
- Node manager: modified reads dispatch + page via the #669 continuation store
  (HistoryContinuationState gains HistoryCursorKind + LastModifiedId), gated by
  the existing HistoryRead decision; session principal / key id passed to the
  updater. IsDeleteModified stays unsupported.
- Client.CLI: historyread --modified.
- Tests: node-manager dispatch/paging/cursor binding/principal threading,
  over-the-wire HistoryModifiedData, shared ModifiedValueConformance on both
  backends, SqlServer atomicity (forced trail failure => no change) + v1->v2
  upgrade + retention trail purge.
- Docs: Historian.md (Modified-value history section, schema v2 upgrade note),
  security.md, Client.CLI.md, CLAUDE.md.

tests/Drivers/Historian.Conformance/ConformanceData.cs

@@ -8,6 +8,9 @@ public static class ConformanceData
     /// <summary>Base time: 2026-10-09 08:00:00.1234567 UTC (sub-millisecond ticks on purpose).</summary>
     public static readonly DateTime T0 = new DateTime(2026, 10, 9, 8, 0, 0, DateTimeKind.Utc).AddTicks(1_234_567);
 
+    /// <summary>The HistoryUpdate caller the suite passes to every updater call (recorded in the modification trail).</summary>
+    public static readonly ZB.MOM.WW.OtOpcUa.Core.Abstractions.HistoryUpdateContext Editor = new("conformance-editor", "k-conf");
+
     /// <summary>Good.</summary>
     public const uint Good = 0x00000000u;
     /// <summary>Bad (generic).</summary>