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.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/HistoryPaging.cs

@@ -31,11 +31,20 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
 ///     prior pages and must be dropped from the head of the next page (tie de-dup).
 /// </param>
 /// <param name="NumValuesPerNode">The client's per-page cap; re-applied to every resumed page.</param>
-/// <param name="IsEvents">
-///     <c>true</c> for an EVENT HistoryRead cursor (Gitea #669): <paramref name="Tagname"/> then carries the
-///     registered historian event-source name and the boundary fields are over <c>EventTimeUtc</c>. A cursor
-///     is only ever resumed by the read kind that issued it, so a raw point presented to an events read
-///     (or vice versa) is a <c>BadContinuationPointInvalid</c> miss.
+/// <param name="Kind">
+///     Which read issued the cursor (Gitea #669 / #668). A cursor is only ever resumed by the read kind that issued it, so a
+///     raw point presented to an events or modified read (or any other mismatch) is a <c>BadContinuationPointInvalid</c>
+///     miss. <see cref="HistoryCursorKind.Events"/>: <paramref name="Tagname"/> carries the registered historian
+///     event-source name and the boundary fields are over <c>EventTimeUtc</c>.
+/// </param>
+/// <param name="LastModifiedId">
+///     <see cref="HistoryCursorKind.Modified"/> only: the <c>ModifiedId</c> of the last returned modification. With
+///     <paramref name="NextStartUtc"/> (its source time) it is the exclusive resume key; <paramref name="BoundarySkipCount"/>
+///     is unused because the key is unique.
+/// </param>
+/// <param name="WindowStartUtc">
+///     <see cref="HistoryCursorKind.Modified"/> only: the normalised window start, so a resumed page keeps the original
+///     window and direction (the modified read resumes by key, not by moving the window start).
 /// </param>
 internal sealed record HistoryContinuationState(
     string Tagname,
@@ -43,7 +52,22 @@ internal sealed record HistoryContinuationState(
     DateTime EndUtc,
     int BoundarySkipCount,
     uint NumValuesPerNode,
-    bool IsEvents = false);
+    HistoryCursorKind Kind = HistoryCursorKind.Raw,
+    long LastModifiedId = 0,
+    DateTime WindowStartUtc = default);
+
+/// <summary>The HistoryRead kind that issued a <see cref="HistoryContinuationState"/>.</summary>
+internal enum HistoryCursorKind
+{
+    /// <summary>HistoryReadRaw (values).</summary>
+    Raw,
+
+    /// <summary>HistoryReadEvents (#669).</summary>
+    Events,
+
+    /// <summary>HistoryReadRaw with <c>IsReadModified = true</c> — the Part 11 modification trail (#668).</summary>
+    Modified,
+}
 
 /// <summary>
 ///     Pure server-side continuation-point paging decisions for the count-capped variable-history arms