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/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/NodeManagerHistoryReadTests.cs

@@ -175,10 +175,13 @@ public sealed class NodeManagerHistoryReadTests : IDisposable
         await host.DisposeAsync();
     }
 
-    /// <summary>Raw with IsReadModified=true ⇒ BadHistoryOperationUnsupported (we don't serve modified
-    /// history); the source is never invoked.</summary>
+    /// <summary>
+    ///     #668: Raw with IsReadModified=true on a backend that keeps no modification trail (the recording fake has no
+    ///     ReadModifiedAsync override, like Gateway) ⇒ BadHistoryOperationUnsupported, and the RAW read is never
+    ///     invoked in its place.
+    /// </summary>
     [Fact]
-    public async Task Raw_read_modified_yields_BadHistoryOperationUnsupported()
+    public async Task Raw_read_modified_on_a_backend_without_a_trail_yields_BadHistoryOperationUnsupported()
     {
         var (host, server) = await BootAsync();
         var nm = server.NodeManager!;
@@ -197,10 +200,16 @@ public sealed class NodeManagerHistoryReadTests : IDisposable
             IsReadModified = true,
         };
 
-        var (_, errors) = InvokeHistoryRead(server, nm, details, nodeId);
+        var (results, errors) = InvokeHistoryRead(server, nm, details, nodeId);
 
         errors[0].StatusCode.Code.ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
-        fake.LastCall.ShouldBeNull();
+        results[0].StatusCode.Code.ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
+        fake.LastCall.ShouldBeNull(); // the raw read is never served in place of a modified one
+
+        // The Null default (no historian configured) answers the same.
+        nm.HistorianDataSource = NullHistorianDataSource.Instance;
+        InvokeHistoryRead(server, nm, details, nodeId).Item2[0].StatusCode.Code
+            .ShouldBe(StatusCodes.BadHistoryOperationUnsupported);
 
         await host.DisposeAsync();
     }