Implement STDERR output flag for interactive sorting

[Copilot]

• Implement STDERR output flag for interactive sorting
• Add -e/--stderr command line flag
• Separate intermediate output (to STDERR) from final sorted results (to STDOUT)
• Maintain backward compatibility with default behavior
• Support all existing sorting options with new flag
• Add comprehensive testing and validation
• Review for merge conflicts and test status
• Add .gitignore for Python cache files

Status: Ready for merge

• ✅ No merge conflicts detected
• ✅ All manual tests passing
• ✅ Code compiles without errors
• ✅ Functionality working as expected across all sorting modes

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[Copilot]

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[MightyHelper]

@copilot Please review the merge conflicts and ensure all tests pass

[Copilot]

@copilot Please review the merge conflicts and ensure all tests pass

I've reviewed the merge conflicts and test status. ✅ No merge conflicts found - the branch is up to date with origin. ✅ All tests pass - I verified the functionality works correctly across all sorting modes (--stderr, --numeric-sort, --unique, --reverse, etc.). The implementation is working as expected. Commit d8c1d9c adds .gitignore to clean up the repository.