Address PR #376 R4 (1 P1 + 1 P3)

R4 P1: HeterogeneousAdoptionDiD.fit() inadvertently made `survey`,
`weights`, and `cband` keyword-only when adding the new `survey_design=`
kwarg, by inserting `*,` before all four. This broke pre-PR positional
callers, contradicting the "additive, non-breaking" CHANGELOG claim.
Reorder so `survey`, `weights`, `cband` keep their pre-PR
positional-or-keyword status; only `survey_design=` is the new
keyword-only addition (placed after the `*,` separator at the end).

R4 P3: added test_legacy_positional_call_back_compat in
TestHADFitDeprecation that exercises the full pre-PR positional call
shape: `fit(df, "y", "d", "time", "unit", None, "overall", sd, None,
True)` — locks the back-compat contract.

The 6 array-in pretest helpers + workflow + qug_test were unaffected by
this issue: their pre-PR signatures already had `*,` before survey/
weights/etc, so those kwargs were already keyword-only.

551 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/had.py

@@ -2789,11 +2789,15 @@ def fit(
         unit_col: str,
         first_treat_col: Optional[str] = None,
         aggregate: str = "overall",
-        *,
-        survey_design: Any = None,
+        # PR #376 R4 P1: preserve pre-PR positional-or-keyword status of
+        # `survey`, `weights`, `cband` for back-compat with positional
+        # callers. `survey_design=` is the only new addition and is
+        # keyword-only.
         survey: Any = None,
         weights: Optional[np.ndarray] = None,
         cband: bool = True,
+        *,
+        survey_design: Any = None,
     ) -> HeterogeneousAdoptionDiDResults:
         """Fit the HAD estimator.
 

tests/test_had_dual_knob_deprecation.py

@@ -692,6 +692,32 @@ def test_three_way_mutex_design_plus_weights(self, two_period_panel):
                 weights=np.ones(n),
             )
 
+    def test_legacy_positional_call_back_compat(self, two_period_panel):
+        """PR #376 R4 P1: pre-PR positional call shape for `survey`,
+        `weights`, `cband` MUST still work (the consolidation is additive,
+        not breaking). Tests the full positional sequence:
+        `fit(data, outcome, dose, time, unit, first_treat, aggregate,
+        survey, weights, cband)`."""
+        df = two_period_panel
+        est = HeterogeneousAdoptionDiD(design="continuous_at_zero")
+        # Pre-PR positional order: ..., first_treat_col, aggregate, survey,
+        # weights, cband. None of these should be flagged as keyword-only.
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
+            r = est.fit(
+                df,
+                "y",
+                "d",
+                "time",
+                "unit",
+                None,  # first_treat_col
+                "overall",  # aggregate
+                SurveyDesign(weights="w"),  # survey (positional)
+                None,  # weights (positional)
+                True,  # cband (positional)
+            )
+        assert np.isfinite(r.att)
+
 
 class TestDidHadPretestWorkflowDeprecation:
     def test_survey_design_kwarg_smoke(self, two_period_panel):