Expose actionable C# top-level symbols

[Widthdom]

Summary

• persist executable C# top-level bodies as file-scoped synthetic functions
• expose stable, generation-bound selectors through outline and inspect, and resolve them in CLI/MCP callees without cross-file collisions
• preserve local-function reference ownership, typed using declarations, legal-whitespace directives, unused-analysis exclusion, and legacy-index upgrade behavior

Validation

• dotnet build CodeIndex.sln --no-restore — 0 warnings, 0 errors
• dotnet test CodeIndex.sln -c Release --no-restore --filter "FullyQualifiedName~Unused|FullyQualifiedName~Issue5164|FullyQualifiedName~RunOutline|FullyQualifiedName~ToolsCall_Outline" — 198 passed on both net8.0 and net9.0
• full Release suite on the implementation baseline — net8.0: 11,857 passed / 9 skipped; net9.0: 11,377 passed / 426 skipped
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj -- check
• canonical index and workspace checks are healthy with current graph, completeness, and fold readiness
• two bounded adversarial-review rounds completed; all eight actionable findings were addressed and their focused regressions pass

Documentation and changelog

• updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in English and Japanese
• added changelog.d/unreleased/5164.added.md

Follow-up candidates

• None.

Fixes #5164