Reject fweight for StackedDiD survey and add regression test from PR #226 review (round 17)

Q-weight composition produces non-integer composed weights, breaking
frequency-weight semantics. Raise ValueError for weight_type='fweight'.

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

Author: igerber

diff_diff/stacked_did.py

@@ -243,6 +243,19 @@ def fit(
             _resolve_survey_for_fit(survey_design, data, "analytical")
         )
 
+        # Reject fweight — Q-weight composition is ratio-valued and breaks
+        # frequency-weight semantics (fweights must be integers)
+        if (
+            survey_design is not None
+            and hasattr(survey_design, "weight_type")
+            and survey_design.weight_type == "fweight"
+        ):
+            raise ValueError(
+                "StackedDiD does not support frequency weights (weight_type='fweight') "
+                "because Q-weight composition produces non-integer composed weights. "
+                "Use weight_type='pweight' (default) or 'aweight' instead."
+            )
+
         # Collect survey design column names for propagation through sub-experiments
         survey_cols: List[str] = []
         if survey_design is not None and isinstance(survey_design, SurveyDesign):

tests/test_survey_phase3.py

@@ -328,6 +328,21 @@ def test_summary_includes_survey(self, staggered_survey_data):
         )
         assert "Survey Design" in result.summary()
 
+    def test_fweight_rejected(self, staggered_survey_data):
+        """StackedDiD should reject fweight since Q-weight composition breaks it."""
+        from diff_diff import StackedDiD
+
+        sd = SurveyDesign(weights="weight", weight_type="fweight")
+        with pytest.raises(ValueError, match="fweight"):
+            StackedDiD().fit(
+                staggered_survey_data,
+                "outcome",
+                "unit",
+                "time",
+                "first_treat",
+                survey_design=sd,
+            )
+
 
 # =============================================================================
 # BaconDecomposition