Migrate state EITC/CTC analyses off split state H5 datasets

PolicyEngine/populace#237 removed release generation for one-file-per-state and one-file-per-district artifacts. The replacement path is the single certified Populace national H5 plus the standard region/scoping machinery in policyengine.py, not direct hf://policyengine/policyengine-us-data/states/... or districts/... files.

GitHub code search still finds split-H5 references in this repo:

• scripts/modal_pipeline.py
• scripts/generate_data.py

Migration checklist:

• Stop constructing or documenting policyengine-us-data/states/*.h5 and policyengine-us-data/districts/*.h5 dataset URIs.
• Use the certified Populace default dataset through policyengine.py, with region filters/scoping for state and congressional-district analyses.
• Add or update a regression test so future state/CD runs do not depend on split regional H5 files.
• Keep historical/generated output unchanged unless it is actively used by the app or docs.

Review scripts/modal_pipeline.py and scripts/generate_data.py for split state or district H5 dataset references, then trace the standard region/scoping machinery in policyengine.py. Add or update a regression test covering state and congressional-district runs; done means those runs use the certified national Populace dataset without split regional H5 URIs.