Fix C# named-argument type references

[Widthdom]

Summary

• suppress C# named-argument labels from type_reference rows without name-based exceptions
• preserve value-side types from named out declarations, explicitly typed lambdas and anonymous methods, typed LINQ range variables, and property subpatterns
• add extractor and persisted references/deps regressions covering positional, reordered, attribute, constructor, nested, multiline, alias, label, nullable, and ternary cases

Root cause

The shared declaration-type scanner treated invocation argument fragments and comma-terminated continuation lines as declarations, so a leading name: label could be emitted as a type reference and resolve to an unrelated same-named symbol.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore — 0 warnings, 0 errors
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore -p:UseSharedCompilation=false --filter "FullyQualifiedName~Issue4830|FullyQualifiedName~Issue4832|FullyQualifiedName~Issue4833" — .NET 8/9: 6 passed each
• dotnet test CodeIndex.sln -c Release --no-build --no-restore -p:UseSharedCompilation=false — .NET 8: 10,515 passed / 7 skipped; .NET 9: 10,042 passed / 419 skipped; 0 failed
• dotnet run --project tools/CodeIndex.Changelog -- check — validated 8 fragments
• full local index rebuild and status --check --json — fresh, 1,210 files, 0 warnings/errors
• references overwrite --exact --lang csharp --json — 0 references
• Codex adversarial review — 2 rounds completed; all actionable findings were fixed and covered by regression fixtures

Documentation and changelog

• DEVELOPER_GUIDE.md
• TESTING_GUIDE.md
• changelog.d/unreleased/4833.fixed.md

Follow-up candidates

None.

Fixes #4833