Add housing assistance count calibration targets

[MaxGhenis]

Summary

• add a HUD Picture of Subsidized Households ETL that loads national and state assisted-household count targets
• include housing assistance household-count targets in calibration target_config
• make take-up-affected constraint variables, including housing_assistance, participate in matrix precomputation and county-dependent take-up rerandomization

Validation

• uv run pytest tests/unit/test_etl_housing_assistance.py tests/unit/datasets/test_cps_takeup.py tests/unit/calibration/test_target_config.py tests/unit/calibration/test_unified_matrix_builder.py -q
• uv run ruff check policyengine_us_data/db/etl_housing_assistance.py policyengine_us_data/calibration/unified_matrix_builder.py policyengine_us_data/utils/takeup.py tests/unit/test_etl_housing_assistance.py tests/unit/calibration/test_target_config.py tests/unit/calibration/test_unified_matrix_builder.py
• live HUD 2024 extract smoke test: 51 states/DC, 4,479,550 assisted households