Fix scoped C# dry-run expansion

[Widthdom]

Summary

• make scoped C# index --dry-run apply the same static-interface and qualified member-read workspace expansion used by execution
• include expanded candidates and changed-between stale C# cleanup in projected counts, limits, samples, and mutation estimates
• expose explicit authoritative/unavailable metadata for capped, unreadable, legacy-schema, or incomplete preflight cases while preserving strict non-mutation
• add paired dry-run/execution regression coverage for --files, --commits, and --changed-between

Validation

• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check
• Release solution build: 0 warnings, 0 errors
• Issue Make git-scoped index dry-run include C# workspace expansion #5225 tests: net8.0 14 passed; net9.0 14 passed
• Run_DryRun tests: net8.0 69 passed; net9.0 66 passed, 2 skipped
• full Release net8.0 suite: 12,121 passed, 9 skipped; one unrelated allocation-budget comparison failed once and passed its isolated retry
• full Release net9.0 suite: 11,637 passed, 431 skipped, 0 failed
• Codex adversarial review: two rounds; all five actionable findings were addressed
• final repository and workspace index health checks passed

Documentation and changelog

• updated README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md
• added bilingual fragment changelog.d/unreleased/5225.fixed.md

Follow-ups

None.

Fixes #5225