Add entity variables and convenience functions for V1 economy output parity

[anth-volk]

Context

Phase 2 of the simulation/report V2 migration requires the API to compute all economy-wide outputs that V1 currently returns (poverty by demographics, inequality, budget summary, intra-decile impacts). The computation library (policyengine.py) needs additional entity variables surfaced in the simulation output and convenience functions to support these.

Changes needed

Entity variable additions

• poverty_type field on Poverty output class
• is_male added to UK and US person entity_variables (for gender poverty)
• race added to US person entity_variables (for racial poverty)
• household_state_income_tax added to US tax_unit entity_variables (for state tax budget line)
• household_income_decile and household_count_people added to UK and US household entity_variables (for intra-decile computation)

Convenience functions

• calculate_us_poverty_by_age() / calculate_uk_poverty_by_age()
• calculate_us_poverty_by_gender() / calculate_uk_poverty_by_gender()
• calculate_us_poverty_by_race() (US only)

Target branch

app-v2-migration