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.

docs/historian/sqlserver/002_sample_modified.sql

@@ -0,0 +1,29 @@
+-- OtOpcUa SQL Server historian backend - schema version 2: OPC UA Part 11 modified-value history (Gitea #668).
+-- $(Schema) is substituted by SqlServerSchemaBootstrapper (identifier-validated) BEFORE the script is split on GO lines.
+-- Every statement is guarded so a re-run is a no-op.
+--
+-- One row per sample a HistoryUpdate inserted, replaced or deleted, carrying the PRIOR value (all NULL for an
+-- insert). Rows are written by the SAME statement that changes hist.Sample (OUTPUT ... INTO), so a failed update
+-- leaves no audit row and a committed one always has its row.
+-- WARNING: OUTPUT ... INTO forbids, on its target table: enabled triggers, either side of a FOREIGN KEY, and CHECK
+-- constraints. Do not add any of those to SampleModified, or every HistoryUpdate fails.
+IF OBJECT_ID(N'[$(Schema)].[SampleModified]', N'U') IS NULL
+CREATE TABLE [$(Schema)].[SampleModified] (
+  TagId int NOT NULL,
+  SourceTimeUtc datetime2(7) NOT NULL,
+  ModifiedId bigint IDENTITY(1,1) NOT NULL,
+  ModificationTimeUtc datetime2(7) NOT NULL,
+  UpdateType tinyint NOT NULL,
+  Principal nvarchar(200) NOT NULL,
+  ApiKeyId nvarchar(200) NULL,
+  ValueKind tinyint NULL,
+  NumValue float NULL,
+  IntValue bigint NULL,
+  StrValue nvarchar(max) NULL,
+  StatusCode int NULL,
+  ServerTimeUtc datetime2(7) NULL,
+  CONSTRAINT PK_SampleModified PRIMARY KEY CLUSTERED (TagId, SourceTimeUtc, ModifiedId))
+GO
+IF NOT EXISTS (SELECT 1 FROM [$(Schema)].[SchemaVersion] WHERE Version = 2)
+INSERT [$(Schema)].[SchemaVersion] (Version, AppliedUtc) VALUES (2, SYSUTCDATETIME())
+GO