Restore C# partial-family unused detection

[Widthdom]

Summary

• Restore cross-file fallback usage detection for private members in C# partial types, including nested partial declarations.
• Match sibling declarations by fully qualified logical type identity, semantic containing-type shape, and generic arity.
• Restrict fallback evidence to the matched peer type's source range so unrelated same-file types cannot suppress genuine unused findings.
• Add one shared three-file regression fixture across regular JSON, compact JSON, --by-bucket, and --actionable output.

Root cause

Nested type matching omitted qualified ancestor paths, while the fallback could also flatten distinct generic containing types and count identifier occurrences outside the matched peer declaration.

Validation

• Debug and Release builds: 0 warnings, 0 errors
• Focused Fix unused analysis for partial-class handlers and field-like symbols #3673 / Restore cross-file partial-class usage detection in unused analysis #4834 tests: 3 passed on both net8.0 and net9.0
• RunUnused suite: 41 passed on both net8.0 and net9.0
• Full Release suite:
  • net8.0: 10,514 passed, 7 skipped, 0 failed
  • net9.0: 10,041 passed, 419 skipped, 0 failed
• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• Local index refreshed; index_matches_workspace: true
• Codex adversarial review: 2 rounds. The generic-arity collision and peer-file source-range findings were both fixed and covered by the shared regression fixture.

Documentation and changelog

• Updated USER_GUIDE.md and TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/4834.fixed.md, referencing the Fix unused analysis for partial-class handlers and field-like symbols #3673 regression.

Follow-up candidates

• None.

Fixes #4834