fix(historian): #683 review — a refused tag ADD INDEX only warns; document index trade-offs

- Bootstrapper: an ALTER ... ADD INDEX refused up front (e.g. no DDL rights)
  logs a Warning and lets the bootstrap succeed (the index is optional); a
  WAL-apply failure remains unobserved, like the TTL ALTER — documented.
  Integration test (VARCHAR tag => refused ALTER) fails with the catch removed.
- Historian.md: a dominant tag can read ~20% slower on long windows (single-
  threaded index scan vs parallel JIT filter); O3 writes rewrite index files;
  sample_modified.tag / event.source deliberately not indexed, and why.
- CLAUDE.md: the concurrent-HistoryUpdate-delete caveat on the range-first order.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TagIndexTests.cs

@@ -83,6 +83,26 @@ public sealed class TagIndexTests(QuestDbFixture fx)
         (await b.TagIndexedAsync()).ShouldBeTrue();
     }
 
+    [Fact]
+    public async Task A_refused_ADD_INDEX_does_not_fail_the_bootstrap()
+    {
+        fx.SkipIfUnavailable();
+        // A sample table whose tag column cannot be indexed (VARCHAR, not SYMBOL): the ALTER is refused up front, which
+        // stands in for any refusal (e.g. no DDL rights). The index is optional, so the bootstrap must still succeed.
+        var p = fx.UniquePrefix();
+        var o = fx.Options(p);
+        var conn = new QuestDbConnection(o);
+        await using (var c = await conn.OpenAsync(Ct))
+        {
+            var ddl = QuestDbTables.CreateSample(p, null).Replace("tag SYMBOL INDEX", "tag VARCHAR", StringComparison.Ordinal);
+            await using var cmd = conn.CreateCommand(c, ddl);
+            await cmd.ExecuteNonQueryAsync(Ct);
+            await using var alter = conn.CreateCommand(c, QuestDbTables.AddTagIndex(conn.SampleTable));
+            await Should.ThrowAsync<Exception>(() => alter.ExecuteNonQueryAsync(Ct)); // precondition: really refused
+        }
+        await new QuestDbSchemaBootstrapper(conn, o).EnsureAsync(Ct);
+    }
+
     [Fact]
     public async Task The_data_range_seeks_the_tag_index()
     {