Precompute tax unit construction inputs

[MaxGhenis]

Summary

• precompute tax-unit income, disability, and student inputs once before household iteration
• keep per-household fallback behavior for direct helper use
• add changelog entry

Tests

• uv run ruff format --check policyengine_us_data/datasets/cps/tax_unit_construction.py
• uv run ruff check policyengine_us_data/datasets/cps/tax_unit_construction.py
• uv run python -m pytest tests/unit/datasets/test_cps_tax_unit_construction.py tests/unit/datasets/test_acs_tax_unit_construction.py -q