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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs

@@ -7,7 +7,7 @@ using ZB.MOM.WW.OtOpcUa.Runtime.Historian;
 namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer;
 
 /// <summary>
-///     Deletes <c>hist.Sample</c> / <c>hist.Event</c> rows older than the retention window, in small
+///     Deletes <c>hist.Sample</c> / <c>hist.SampleModified</c> / <c>hist.Event</c> rows older than the retention window, in small
 ///     <c>DELETE TOP(n)</c> batches (under the lock-escalation threshold). Samples are purged per tag so each
 ///     delete is a seek on the clustered key <c>(TagId, SourceTimeUtc)</c>; events seek on <c>CX_Event_Time</c>.
 ///     A session <c>sp_getapplock</c> (timeout 0) serialises purgers that share one database; a loser skips.
@@ -53,7 +53,7 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
                 return [];
             }
 
-            long samples = 0;
+            long samples = 0, modified = 0;
             var tagIds = new List<int>();
             await using (var cmd = _conn.CreateCommand(conn, $"SELECT TagId FROM {_conn.Q("Tag")}"))
             await using (var r = await cmd.ExecuteReaderAsync(ct).ConfigureAwait(false))
@@ -64,13 +64,23 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
                 samples += await DeleteLoopAsync(conn,
                     $"DELETE TOP (@n) FROM {_conn.Q("Sample")} WHERE TagId = @id AND SourceTimeUtc < @cutoff",
                     cutoff, id, ct).ConfigureAwait(false);
+                // The Part 11 modification trail (#668) ages out on the SAME source-time cutoff as the samples it
+                // describes; a purge never writes to it (no trigger, no OUTPUT INTO).
+                modified += await DeleteLoopAsync(conn,
+                    $"DELETE TOP (@n) FROM {_conn.Q("SampleModified")} WHERE TagId = @id AND SourceTimeUtc < @cutoff",
+                    cutoff, id, ct).ConfigureAwait(false);
             }
 
             var events = await DeleteLoopAsync(conn,
                 $"DELETE TOP (@n) FROM {_conn.Q("Event")} WHERE EventTimeUtc < @cutoff",
                 cutoff, null, ct).ConfigureAwait(false);
 
-            return [new RetentionPurgeResult("Sample", samples), new RetentionPurgeResult("Event", events)];
+            return
+            [
+                new RetentionPurgeResult("Sample", samples),
+                new RetentionPurgeResult("SampleModified", modified),
+                new RetentionPurgeResult("Event", events),
+            ];
         }
         finally
         {