cleanup: remove diff.txt and prevent re-adding

Removed transitional artifact file used during restructuring:
- diff.txt tracked import path changes during migration
- All imports now updated to relative paths
- All 41 tests passing

Updated .gitignore to prevent accidental re-adding.

Repository now clean and ready for Phase 6 (documentation).

Author: miranov25

UTILS/dfextensions/groupby_regression/.gitignore

@@ -37,3 +37,6 @@ diff.txt
 
 # Transitional artifacts
 diff.txt
+
+# Transitional artifacts
+diff.txt