[codex] Backport congressional district outcome percentages to 0.x

[MaxGhenis]

Summary

• add winner, loser, and no-change percentages to legacy 0.x congressional_district_impact rows
• compute those shares using people weights when available, with a household-weight fallback
• add regression coverage for the new percentages and fallback behavior

Validation

• python3 -m py_compile policyengine/outputs/macro/comparison/calculate_economy_comparison.py tests/country/test_us_congressional_districts.py tests/fixtures/simulation.py
• black -l 79 --check policyengine/outputs/macro/comparison/calculate_economy_comparison.py tests/country/test_us_congressional_districts.py tests/fixtures/simulation.py
• direct function-level smoke check for people-weighted and fallback percentages

Notes

• local pytest tests/country/test_us_congressional_districts.py is blocked here by a pre-existing YAML parse error in the installed policyengine_us package before test collection starts, so GitHub CI is the authoritative test signal for this branch.