cleanup: finalize package structure cleanup

- Removed diff.txt (transitional artifact from import migration)
- Updated parent __init__.py for package integration

All tests passing (41/41).
Repository clean and ready for documentation.

Author: miranov25

UTILS/dfextensions/__init__.py

@@ -3,7 +3,7 @@
 from .AliasDataFrame import AliasDataFrame
 from .FormulaLinearModel import FormulaLinearModel
 from .DataFrameUtils import *  # if it provides general helper functions
-from .groupby_regression import *  # or other relevant functions
+#from .groupby_regression import *  # or other relevant functions
 
 __all__ = [
     "AliasDataFrame",