Add ability to run impacts over subnational regions

[anth-volk]

Summary

Add support for running policy impact calculations at subnational geographic levels (states, congressional districts, local authorities, constituencies, and places/cities).

Motivation

PolicyEngine needs to support regional impact analysis for policy reforms at different geographic granularities:

• US: States (51), Congressional Districts (436), and Places/Cities (333)
• UK: Countries (4), Regions (12), Local Authorities (~370), and Constituencies (~650)

Implementation

This feature adds:

1. Region model (src/policyengine/core/region.py):

   • Region class with code, label, type, and optional dataset paths
   • RegionRegistry for O(1) lookups by code or type
   • Support for HuggingFace and GCS dataset paths

2. Country-specific region definitions:

   • US: States, congressional districts, and places with FIPS codes
   • UK: Countries, regions, local authorities, and constituencies

3. Integration with tax benefit models:

   • TaxBenefitModelVersion.regions property
   • Region registries accessible via model versions

4. Test coverage:

   • Unit tests for Region and RegionRegistry
   • Country-specific tests for US and UK region data
   • Regional impact calculation verification script

Testing

A test script (scripts/test_regional_impacts.py) verifies that:

• Simulations run correctly at national, state, and district levels
• Budgetary impacts are calculated correctly
• Subnational impacts are proportional to population shares

Related

This enables the app v2 to display regional impacts for policy reforms.