Relax itemized deduction release gate

[MaxGhenis]

Summary

• allow the national itemized deduction total critical release gate to use the same 15% cap as other broad credit/deduction critical checks
• keep SALT at the tighter 10% cap
• update contract tests to document the intended cap split

Validation

• ./.venv/bin/python3 -m pytest packages/populace-data/tests/test_contract.py -q
• ./.venv/bin/ruff check packages/populace-data/src/populace/data/contract.py packages/populace-data/tests/test_contract.py