Track C# lambda capture references

[Widthdom]

Summary

• Emit C# capture reference edges when a lambda reads an enclosing local variable.
• Include capture in graph traversal so callers/impact can surface closure dependencies.
• Add regression coverage for lambda parameter shadowing and same-named method scopes.

Documentation and changelog

• Updated DEVELOPER_GUIDE.md reference-kind filtering matrix in English and Japanese.
• Added changelog.d/unreleased/2061.fixed.md.

Fixes #2061

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~ReferenceExtractorTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReferenceExtractorTests|FullyQualifiedName~DbReaderTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "CSharpLambdaCapture|ReferenceKindMatrix_CallersIncludesCSharpLambdaCaptures"
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• Adversarial review: No blocking/actionable issues found.

Follow-up candidates

• Existing/unrelated full-suite failures observed locally are tracked by Trimmed publish test can fail with ILLink AccessViolationException on macOS #2586, Rust associated type defaults are not emitted for trait symbols #2603, and Concurrent freshness snapshot test can hit nested SQLite transaction #2607.