Document and enforce symbol kind taxonomy

[Widthdom]

Summary

• Documented the symbol/reference kind taxonomy and centralized registered kinds in SymbolKindCatalog.
• Added schema and writer validation for symbol/reference kind columns.
• Reclassified C# operator overloads/conversions as operator symbols and updated callable/static-interface lookup paths.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameDatabaseTests|FullyQualifiedNameSymbolExtractorTests|FullyQualifiedNameReferenceExtractorTests|FullyQualifiedNameIndexCommandRunnerTests|FullyQualifiedNameMcpServerTests|FullyQualifiedNameDbReaderTests|FullyQualifiedName~QueryCommandRunnerTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex adversarial review: No blocking/actionable issues found.

Follow-up

• Related discovered issue: Investigate cdidx index hang leaving unfinished batch marker #2693

Fixes #1762
Fixes #1691
Fixes #1965