Add congressional district breakdowns to state-level simulations

[anth-volk]

Fixes #216

Summary

Adds congressional district-level breakdowns to US state-level simulation results, mirroring the existing UK parliamentary constituency breakdown functionality.

Changes

Core Implementation

policyengine/outputs/macro/single/calculate_single_economy.py

• Added congressional_district_geoid: List[int] | None to SingleEconomy model
• Added calculate_congressional_district_geoid() method to GeneralEconomyTask

policyengine/outputs/macro/comparison/calculate_economy_comparison.py

• Added USCongressionalDistrictImpact model (per-district results)
• Added USCongressionalDistrictBreakdownWithValues model (list of districts)
• Added us_congressional_district_breakdown() function using identical formulas to UK constituencies
• Added congressional_district_impact field to EconomyComparison
• Added geoid_to_district_name() helper to convert SSDD format to "GA-05" style names
• Added STATE_FIPS_TO_ABBREV mapping

Tests

tests/country/test_us_congressional_districts.py (new file)

• 16 tests following given-when-then naming pattern
• Tests for geoid conversion, breakdown calculation, edge cases
• Integration test verifying real GA dataset works

tests/fixtures/simulation.py

• Added create_mock_single_economy() helper
• Added mock fixtures for GA districts, multi-state districts, empty/null cases

tests/conftest.py and tests/fixtures/__init__.py

• Added to properly expose fixtures

Output Format

{
  "congressional_district_impact": {
    "districts": [
      {
        "district": "RI-01",
        "average_household_income_change": 320.73,
        "relative_household_income_change": 0.001944
      },
      {
        "district": "RI-02",
        "average_household_income_change": 199.13,
        "relative_household_income_change": 0.001132
      }
    ]
  }
}

Verification

Tested with Rhode Island (2 districts) - making CTC fully refundable:

• ✓ Returns exactly 2 districts (RI-01, RI-02)
• ✓ District names formatted correctly
• ✓ Numerical values are floats
• ✓ Relative changes are decimals (not percentages)

Formula Confirmation

The district-level calculations use identical formulas to UK parliamentary constituencies:

Metric	Formula
average_household_income_change	(reform.sum() - baseline.sum()) / baseline.count()
relative_household_income_change	reform.sum() / baseline.sum() - 1

Test Plan

• Unit tests for geoid_to_district_name()
• Unit tests for us_congressional_district_breakdown() with mocked data
• Integration test with real GA state dataset
• Manual verification with RI state simulation