Fix C# positional record search attribution

[Widthdom]

Summary

• persist precise C# positional-record component spans so record prefixes, components, base arguments, and body members keep the correct enclosing symbol
• select enclosing symbols from the displayed primary match line and raw source column, including per-result exact-name normalization and multi-hit fallback behavior
• preserve legacy Java/Kotlin record-component signatures while limiting offset-preserving comment padding to C#
• add reader, CLI, extractor-contract, grouping, multiline, same-line, and cross-language regression coverage plus bilingual documentation

Validation

• make lint
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj -- check
• dotnet build CodeIndex.sln -c Release --no-restore
• full Release suite before the final guard-path adjustment: net9.0 11,166 passed / 423 skipped with the guard-path failure; net8.0 11,644 passed / 7 skipped with the same failure plus Stabilize Docker entrypoint contract timeout under full-suite load #5132
• final Release regression set for Attribute C# record declaration search hits to the record type instead of positional members #5095, exact language normalization, all guard-search cases, and Java/Kotlin signature compatibility: 21 passed on net8.0 and 21 passed on net9.0
• isolated unrelated full-suite timeout rerun passed on net8.0 and remained skipped on net9.0
• two codex review --base origin/main adversarial-review rounds completed; all findings were addressed
• root and workspace index health checks pass at the final commit

Documentation

• updated the English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md
• added changelog.d/unreleased/5095.fixed.md

Follow-up

• Git timeout regression test cannot reach diff-tree after single-commit validation #5130
• Stabilize Docker entrypoint contract timeout under full-suite load #5132

Fixes #5095