Add decomposed marginal tax rate variables (federal, state, FICA)

[PavelMakarchuk]

Summary

PolicyEngine currently has a single marginal_tax_rate variable that computes the all-in effective marginal rate based on household_net_income (including benefit phaseouts). For comparison with TAXSIM-35, we need separate marginal tax rate variables for each tax component:

• federal_marginal_tax_rate — marginal change in federal income tax
• state_marginal_tax_rate — marginal change in state income tax
• fica_marginal_tax_rate — marginal change in FICA liability

Context

TAXSIM-35 outputs three separate marginal rates (frate, srate, ficar), computed by perturbing wages and measuring the change in each tax component independently. PE's existing marginal_tax_rate measures something fundamentally different — the fraction of marginal income that doesn't increase household net income, which includes benefit cliffs, transfer phaseouts, etc.

These component-level rates would enable apples-to-apples comparison of marginal rates in policyengine-taxsim and would also be independently useful for policy analysis (e.g., showing how much of a marginal rate comes from federal vs. state taxes).

Suggested approach

Follow the same counterfactual pattern as marginal_tax_rate but measure Δincome_tax, Δstate_income_tax, and Δemployee_payroll_tax separately instead of Δhousehold_net_income.