refactor: move files to package structure (preserve history)

Moved with git mv to preserve full commit history:
- groupby_regression.py → groupby_regression/
- groupby_regression_optimized.py → groupby_regression/
- test_*.py → groupby_regression/tests/
- bench_*.py → groupby_regression/benchmarks/
- *.md → groupby_regression/docs/

No functional changes. History preserved with --follow.

Files can be tracked across rename with:
  git log --follow <file>

Author: miranov25