fix: address CI review round 4 - panel=False guard, drop covariate_effects block

- P1: reject panel=False in data_generator_kwargs for panel-only estimators
  (TWFE, SA, Imputation, TwoStage, Stacked, Efficient); only CS supports
  repeated cross-sections under survey_config
- P2: remove over-broad covariate_effects rejection (it affects baseline
  outcomes and ICC calibration, not realized population_att)
- Add regression tests: panel=False rejected for TWFE, allowed for CS(panel=False)

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

Author: igerber

diff_diff/power.py

@@ -1943,12 +1943,16 @@ def simulate_power(
                 f"the input treatment_effect under covariate-interaction "
                 f"heterogeneity, which would make bias/coverage/RMSE misleading."
             )
-        if "covariate_effects" in data_gen_kwargs:
-            raise ValueError(
-                "covariate_effects is not supported with survey_config. "
-                "Custom covariate effects can shift the realized population "
-                "ATT away from the input treatment_effect."
-            )
+        # Block panel=False for panel-only estimators. Only
+        # CallawaySantAnna supports repeated cross-sections.
+        if not data_gen_kwargs.get("panel", True):
+            _RCS_SUPPORTED = frozenset({"CallawaySantAnna"})
+            if estimator_name not in _RCS_SUPPORTED:
+                raise ValueError(
+                    f"panel=False (repeated cross-sections) is not supported "
+                    f"with {estimator_name} under survey_config. Only "
+                    f"{sorted(_RCS_SUPPORTED)} support repeated cross-sections."
+                )
 
     # SyntheticDiD placebo variance requires n_control > n_treated.
     # Check after merging data_generator_kwargs so overrides of n_treated

tests/test_power.py

@@ -2422,18 +2422,31 @@ def test_survey_rejects_te_covariate_interaction(self):
                 **_SIM_KW,
             )
 
-    def test_survey_rejects_covariate_effects(self):
-        """covariate_effects rejected (can shift population ATT)."""
-        with pytest.raises(ValueError, match="covariate_effects"):
+    def test_survey_rejects_panel_false_for_panel_only(self):
+        """panel=False rejected for panel-only estimators (e.g., TWFE)."""
+        with pytest.raises(ValueError, match="panel=False.*not supported"):
             simulate_power(
-                CallawaySantAnna(),
-                data_generator_kwargs={"covariate_effects": (0.5, 0.3)},
+                TwoWayFixedEffects(),
+                data_generator_kwargs={"panel": False},
                 survey_config=_SURVEY_CFG,
                 n_simulations=1,
                 seed=42,
                 **_SIM_KW,
             )
 
+    def test_survey_allows_panel_false_for_cs(self):
+        """panel=False allowed for CallawaySantAnna (supports RCS)."""
+        result = simulate_power(
+            CallawaySantAnna(panel=False),
+            treatment_effect=3.0,
+            data_generator_kwargs={"panel": False},
+            survey_config=_SURVEY_CFG,
+            n_simulations=10,
+            seed=42,
+            **_SIM_KW,
+        )
+        assert 0 <= result.power <= 1
+
     # -- Closed-form deff tests --
 
     def test_closed_form_deff_default(self):