Propagate replicate design params through subpopulation, subset_to_units, CS collapse

Carry combined_weights, replicate_scale, replicate_rscales, and mse
through all three helper constructors that build derivative
ResolvedSurveyDesign or SurveyDesign objects:
- SurveyDesign.subpopulation() → new SurveyDesign
- ResolvedSurveyDesign.subset_to_units() → new ResolvedSurveyDesign
- CallawaySantAnna._collapse_survey_to_unit_level() → new ResolvedSurveyDesign

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

Author: igerber

diff_diff/staggered.py

@@ -407,6 +407,10 @@ def _collapse_survey_to_unit_level(resolved_survey, df, unit_col, all_units):
             fay_rho=resolved_survey.fay_rho,
             n_replicates=resolved_survey.n_replicates,
             replicate_strata=resolved_survey.replicate_strata,
+            combined_weights=resolved_survey.combined_weights,
+            replicate_scale=resolved_survey.replicate_scale,
+            replicate_rscales=resolved_survey.replicate_rscales,
+            mse=resolved_survey.mse,
         )
 
     def _precompute_structures(

diff_diff/survey.py

@@ -520,6 +520,10 @@ def subpopulation(
             replicate_method=self.replicate_method,
             fay_rho=self.fay_rho,
             replicate_strata=self.replicate_strata,
+            combined_weights=self.combined_weights,
+            replicate_scale=self.replicate_scale,
+            replicate_rscales=self.replicate_rscales,
+            mse=self.mse,
         )
 
         return new_design, data_out
@@ -611,6 +615,10 @@ def subset_to_units(
             fay_rho=self.fay_rho,
             n_replicates=self.n_replicates,
             replicate_strata=self.replicate_strata,
+            combined_weights=self.combined_weights,
+            replicate_scale=self.replicate_scale,
+            replicate_rscales=self.replicate_rscales,
+            mse=self.mse,
         )
 
     @property