fix(weak-iv): add lowercase aliases for first_stage_f / effective_f

Tests in test_weakiv_tobit.py expect lowercase keys but the dict only
exposed the capital-F versions. Add both for backward compatibility —
no existing callers of the capital-F keys are broken, and the 17
weak-IV tests now pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: brycewang-stanford

src/statspai/diagnostics/weak_iv.py

@@ -479,7 +479,9 @@ def _ar_f(b0: float) -> float:
         "ar_ci": ar_ci,
         "beta_2sls": beta_2sls,
         "first_stage_F": f_first,
+        "first_stage_f": f_first,
         "effective_F": f_eff,
+        "effective_f": f_eff,
         "tF_critical_value": tF_c,
         "n_instruments": k_z,
         "strength": f_result["strength"],