Fix outline parents and GraphQL/heading symbol anchors

[Widthdom]

Summary

• Disambiguate outline parent lookup by qualified symbol path for FindOutlineContainerIndex uses only name matching without containment validation #2016.
• Add GraphQL input-field and union-variant anchors, including extend/directive/comment/multiline union forms, for GraphQL input fields and union variants not indexed as anchors #1825.
• Surface section heading anchors for C# regions, Python module docstrings, and JS/TS @module docblocks for outline lacks section-heading detection (#region, docstrings, markdown headers) #1636.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameSymbolExtractorTests|FullyQualifiedNameDbReaderTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex adversarial review: No actionable findings

Changelog fragments

• changelog.d/unreleased/1636.fixed.md
• changelog.d/unreleased/1825.fixed.md
• changelog.d/unreleased/2016.fixed.md

Fixes #1636
Fixes #1825
Fixes #2016