Merton test fixture references undefined returns variable #4

Summary

tests/test_merton_process.py::simple_returns_data creates normal_returns and jump_returns, but then calls np.random.shuffle(returns) and returns pd.Series(returns) without ever defining returns.

Once the missing statsmodels dependency is installed locally, the focused test errors during fixture setup.

Reproduction

Tested on commit b0c13aa after installing statsmodels locally to get past import collection:

MPLBACKEND=Agg .venv/bin/python -m pytest -q \
  tests/test_merton_process.py::test_merton_process_fit_with_explicit_dt

Actual result

NameError: name 'returns' is not defined

The fixture currently has:

normal_returns = np.random.normal(0, 0.01, int(n * 0.9))
jump_returns = np.random.normal(-0.03, 0.02, int(n * 0.1))
np.random.shuffle(returns)
return pd.Series(returns)

Expected result

The fixture should combine normal_returns and jump_returns before shuffling/returning them, for example by assigning returns = np.concatenate([normal_returns, jump_returns]).