refactor: complete migration to new package structure

Removed obsolete directories and artifacts:
- Deleted optimized/ directory (migrated to package structure)
- Removed diff.txt (transitional artifact)
- Cleaned up old benchmark logs

Package structure now complete:
  groupby_regression/
    ├── __init__.py
    ├── groupby_regression.py (robust)
    ├── groupby_regression_optimized.py (v2/v3/v4)
    ├── tests/ (41 tests)
    ├── benchmarks/ (3 scripts + results)
    └── docs/ (restartContext.md)

Ready for Phase 6 (comprehensive documentation).

Author: miranov25

UTILS/dfextensions/.gitignore

@@ -1,3 +1,8 @@
 
 # Transitional artifacts
 diff.txt
+
+# Transitional artifacts
+diff.txt
+*.log
+groupby_regression_git.log