fix: enforce CS panel-mode alignment between estimator and DGP

P1: validate that CallawaySantAnna.panel matches the survey DGP's panel
flag in both directions:
- CS(panel=True) + DGP panel=False -> rejected
- CS(panel=False) + default DGP (panel=True) -> rejected
- Panel check now runs even with empty data_generator_kwargs

Add regression tests for both misaligned CS combinations.

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

Author: igerber

diff_diff/power.py

@@ -1943,16 +1943,31 @@ def simulate_power(
                 f"the input treatment_effect under covariate-interaction "
                 f"heterogeneity, which would make bias/coverage/RMSE misleading."
             )
-        # 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:
+
+    # Enforce panel-mode alignment between DGP and estimator.
+    # Runs even with empty data_gen_kwargs to catch CS(panel=False) + default DGP.
+    if use_survey_dgp:
+        dgp_panel = data_gen_kwargs.get("panel", True)
+        est_panel = getattr(estimator, "panel", True)
+        if not dgp_panel:
+            if estimator_name != "CallawaySantAnna":
                 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."
+                    f"CallawaySantAnna supports repeated cross-sections."
+                )
+            if est_panel:
+                raise ValueError(
+                    "data_generator_kwargs has panel=False but "
+                    "CallawaySantAnna.panel=True. Use "
+                    "CallawaySantAnna(panel=False) to match."
                 )
+        elif estimator_name == "CallawaySantAnna" and not est_panel:
+            raise ValueError(
+                "CallawaySantAnna(panel=False) requires "
+                "data_generator_kwargs={'panel': False} to generate "
+                "repeated cross-section data."
+            )
 
     # SyntheticDiD placebo variance requires n_control > n_treated.
     # Check after merging data_generator_kwargs so overrides of n_treated

tests/test_power.py

@@ -2435,7 +2435,7 @@ def test_survey_rejects_panel_false_for_panel_only(self):
             )
 
     def test_survey_allows_panel_false_for_cs(self):
-        """panel=False allowed for CallawaySantAnna (supports RCS)."""
+        """panel=False allowed for CallawaySantAnna(panel=False) (supports RCS)."""
         result = simulate_power(
             CallawaySantAnna(panel=False),
             treatment_effect=3.0,
@@ -2447,6 +2447,29 @@ def test_survey_allows_panel_false_for_cs(self):
         )
         assert 0 <= result.power <= 1
 
+    def test_survey_rejects_cs_panel_mismatch_dgp_rcs(self):
+        """CS(panel=True) + DGP panel=False rejected."""
+        with pytest.raises(ValueError, match="CallawaySantAnna.panel=True"):
+            simulate_power(
+                CallawaySantAnna(),  # panel=True by default
+                data_generator_kwargs={"panel": False},
+                survey_config=_SURVEY_CFG,
+                n_simulations=1,
+                seed=42,
+                **_SIM_KW,
+            )
+
+    def test_survey_rejects_cs_panel_mismatch_est_rcs(self):
+        """CS(panel=False) + default DGP (panel=True) rejected."""
+        with pytest.raises(ValueError, match="panel=False.*requires"):
+            simulate_power(
+                CallawaySantAnna(panel=False),
+                survey_config=_SURVEY_CFG,
+                n_simulations=1,
+                seed=42,
+                **_SIM_KW,
+            )
+
     # -- Closed-form deff tests --
 
     def test_closed_form_deff_default(self):