Use mortgage-specific JCT target for MID calibration

[MaxGhenis]

Summary

• switch the JCT mortgage tax expenditure target from broad interest_deduction to mortgage-only deductible_mortgage_interest
• update the legacy loss-matrix path and deprecated JCT test dictionaries to use the same mortgage-specific variable
• add a database-build regression test to ensure the ETL loads the mortgage JCT target with the expected variable and JCT provenance in notes

Testing

• python3 -m py_compile policyengine_us_data/db/etl_national_targets.py policyengine_us_data/utils/loss.py policyengine_us_data/tests/test_database_build.py policyengine_us_data/tests/test_datasets/test_enhanced_cps.py policyengine_us_data/tests/test_datasets/test_sparse_enhanced_cps.py
• ruff format --check policyengine_us_data/db/etl_national_targets.py policyengine_us_data/utils/loss.py policyengine_us_data/tests/test_database_build.py policyengine_us_data/tests/test_datasets/test_enhanced_cps.py policyengine_us_data/tests/test_datasets/test_sparse_enhanced_cps.py
• PYTHONPATH=/Users/maxghenis/.codex-worktrees/mid-jct-pr /Users/maxghenis/.codex-worktrees/policyengine-us-data-mid/.venv/bin/python -m pytest policyengine_us_data/tests/test_database_build.py -k mortgage_specific_variable -q

Context

• follow-up to the merged structural MID work in #633
• keeps compatibility with the currently installed policyengine-us package by using the existing mortgage-only person variable rather than the newer tax-unit structural variable