Add --fail-on-diff for CI gating

[Widthdom]

CLOSES #214

Summary

• add opt-in --fail-on-diff and dedicated exit code 5 when final reportable Added/Removed/Modified entries remain
• decide the CI gate only after reports, audit output, and configured post-processing complete; filtered or ignored differences do not trigger it
• preserve the default success behavior and document/test the parser, help, filtering, artifact generation, and exit-code contract in English and Japanese

Validation

• dotnet build FolderDiffIL4DotNet.sln -c Release --no-restore (0 warnings, 0 errors)
• focused Release tests covering CLI/help/gating: 95 passed
• full Release test suite: 2032 passed, 1 skipped, 0 failed
• manual CLI smoke test: exit code 5 with IL output, Markdown/HTML reports, and audit log generated
• git diff --check

Notes

• dotnet format FolderDiffIL4DotNet.sln --verify-no-changes --no-restore still reports existing repository-wide formatting drift in untouched code, so this PR does not apply an unrelated bulk-format change.
• No screenshots are included because this is a CLI-only behavior change.