Strengthen R0/R1 regression tests per CI feedback

CI AI review on #435:
- P2: exp_p / exp_ci returned from safe_inference() were unused (F841).
  Extended event-study test to assert p_value and conf_int match
  safe_inference output too, not just t_stat. The safe_inference
  contract is joint NaN propagation across the full inference tuple,
  so testing all three jointly is the right invariant.
- P3: placebo test only checked 'at least one finite entry', so a
  silent path-drop on the replicate-weight placebo branch could pass.
  Added explicit assertions that both (0,1,1,1) and (0,1,0,0) are
  present in res.path_placebo_event_study before checking SE/inference.

Author: igerber

tests/test_chaisemartin_dhaultfoeuille.py

@@ -9998,12 +9998,20 @@ def test_paths_of_interest_replicate_weight_per_path_se_finite(self):
             for vals in entry["horizons"].values():
                 if vals["n_obs"] > 0 and np.isfinite(vals["se"]) and np.isfinite(vals["effect"]):
                     exp_t, exp_p, exp_ci = safe_inference(vals["effect"], vals["se"], df=final_df)
-                    matches = vals["t_stat"] == exp_t or (
+                    t_matches = vals["t_stat"] == exp_t or (
                         np.isnan(vals["t_stat"]) and np.isnan(exp_t)
                     )
-                    assert matches, (
-                        "Per-path t_stat does not match safe_inference at the "
-                        f"final df_survey={final_df} — refresh did not propagate"
+                    p_matches = vals["p_value"] == exp_p or (
+                        np.isnan(vals["p_value"]) and np.isnan(exp_p)
+                    )
+                    ci_matches = vals["conf_int"] == exp_ci or all(
+                        np.isnan(a) == np.isnan(b)
+                        for a, b in zip(vals["conf_int"], exp_ci, strict=True)
+                    )
+                    assert t_matches and p_matches and ci_matches, (
+                        "Per-path inference fields do not match safe_inference at "
+                        f"final df_survey={final_df} — refresh did not propagate "
+                        "(t/p/conf_int must update jointly per safe_inference contract)"
                     )
 
     @pytest.mark.slow
@@ -10052,8 +10060,12 @@ def test_paths_of_interest_survey_design_placebo_replicate_weight(self):
                 L_max=3,
                 survey_design=sd,
             )
-        # Sanity-check: placebos for at least one requested path are populated.
+        # Both requested paths must be present in the placebo surface —
+        # a regression that silently drops one requested path would slip
+        # through an "at least one finite entry" check.
         assert res.path_placebo_event_study is not None
+        assert (0, 1, 1, 1) in res.path_placebo_event_study
+        assert (0, 1, 0, 0) in res.path_placebo_event_study
         any_finite_placebo = False
         for entry in res.path_placebo_event_study.values():
             for vals in entry.values():