cleanup: remove diff.txt and prevent re-adding

Removed transitional artifact file used during restructuring:
- diff.txt was tracking import path changes
- All imports now updated and working
- Added to .gitignore to prevent accidental re-adding

All 41 tests passing, benchmark results validated.

Author: miranov25

UTILS/dfextensions/groupby_regression/.gitignore

@@ -34,3 +34,6 @@ diff.txt
 # benchmarks/bench_out/*.txt
 # benchmarks/bench_out/*.csv
 # benchmarks/bench_out/*.png
+
+# Transitional artifacts
+diff.txt