Add structural mortgage interest data support

[MaxGhenis]

Summary

• convert imputed deductible_mortgage_interest and interest_deduction into structural mortgage inputs when the installed policyengine-us exposes the new federal MID variables
• add SCF-backed tax-unit mortgage balance hints in Extended CPS so non-itemizer homeowners can carry structural first/second mortgage balances without changing current-law interest deductions
• keep the path compatibility-gated, preserve residual non-mortgage interest through investment_interest_expense, and add focused tests for the conversion and SCF hint imputation

Testing

• PYTHONPATH=/Users/maxghenis/.codex-worktrees/policyengine-us-mid uv run pytest policyengine_us_data/tests/test_calibration/test_mortgage_interest.py
• PYTHONPATH=/Users/maxghenis/.codex-worktrees/policyengine-us-mid uv run ruff check policyengine_us_data/utils/mortgage_interest.py policyengine_us_data/datasets/cps/extended_cps.py policyengine_us_data/tests/test_calibration/test_mortgage_interest.py policyengine_us_data/datasets/puf/puf.py
• PYTHONPATH=/Users/maxghenis/.codex-worktrees/policyengine-us-mid uv run python -m py_compile policyengine_us_data/utils/mortgage_interest.py policyengine_us_data/datasets/cps/extended_cps.py policyengine_us_data/tests/test_calibration/test_mortgage_interest.py policyengine_us_data/datasets/puf/puf.py

Notes

• the new structural path is a no-op until policyengine-us includes the paired MID variables
• this PR does not rebuild Extended CPS artifacts or rerun the full calibration pipeline