Add federal vs. state budgetary impact to economic_impact_analysis

[MaxGhenis]

Summary

Partitions US reform budgetary impact into federal vs. state shares:

• BudgetaryImpact (federal/state/total) model added to PolicyReformAnalysis
• calculate_budgetary_impact(baseline_sim, reform_sim) -> BudgetaryImpact helper
• Example output in examples/us_budgetary_impact.py

Closes #289.

Formulas

federal = Δ(income_tax) + Δ(payroll_tax) - Δ(federal_benefit_cost)
state   = Δ(state_income_tax)            - Δ(state_benefit_cost)
total   = federal + state

Where federal_benefit_cost and state_benefit_cost are the PE-US aggregates from PolicyEngine/policyengine-us#8076 that sum federal/state shares of Medicaid (FMAP) and CHIP (eFMAP). As more programs gain attribution (SNAP OBBBA FY2028, SSI, etc.), the aggregates grow automatically via the parameter-driven adds list — no changes needed here.

Why here vs. in policyengine-api

The arithmetic is thin sums over microsim outputs. Every consumer — API, notebooks, ad-hoc scripts — needs the same split; putting it behind the .py package means one implementation. The API follow-up (PolicyEngine/policyengine-api#3481) becomes a pass-through.

Test plan

• pytest tests/test_budgetary_impact.py -v — 4 unit tests pass (federal-tax-cut, Medicaid expansion rollback fed/state split, mixed, zero-reform)
• CI (will require PE-US pin bump to release containing #8076; policyengine-us >= 1.XXX.Y)

Scope notes

• Programs that are 100% federal (SSI, LIHEAP, WIC, HCV, school meals) and 100% state (state supplements via household_state_benefits) are not folded in here — this exposes only the shared-funding programs in federal_benefit_cost / state_benefit_cost. Follow-up can add federal_only_benefit_cost and state_only_benefit_cost aggregates if useful; for now users can compute those directly.
• Tax revenue split assumes state_income_tax is fully state (true) and income_tax + payroll_tax is fully federal (true). Not yet modeled: local income taxes, state-level tax credits that show up under federal variables.

Related

• Add federal vs. state cost attribution for benefit programs (Medicaid FMAP, SNAP OBBBA, CHIP eFMAP, etc.) policyengine-us#8075 (scoping)
• Add Medicaid + CHIP federal/state cost attribution via FMAP policyengine-us#8076 (Medicaid + CHIP attribution, merged)
• Expose federal vs. state budgetary impact in compare.py policyengine-api#3481 (API surface, depends on this)
• Show federal vs. state fiscal impact in budget view policyengine-app-v2#999 (UI, depends on API)
• Example notebook: federal vs. state Medicaid fiscal impact analysis-notebooks#131 (example notebook, depends on this)