Assign distinct reform IDs to national tax expenditure targets

[MaxGhenis]

Summary

• assign distinct reform_id values to each national JCT tax expenditure target instead of reusing a single generic reform id
• store those ids directly on the target rows so the reform identity stays stable even if a target variable name changes later
• add a database-build regression test covering the distinct reform ids

Testing

• python3 -m py_compile policyengine_us_data/db/etl_national_targets.py policyengine_us_data/tests/test_database_build.py
• ruff check policyengine_us_data/db/etl_national_targets.py policyengine_us_data/tests/test_database_build.py
• PYTHONPATH=/Users/maxghenis/.codex-worktrees/jct-reform-ids /Users/maxghenis/.codex-worktrees/policyengine-us-data-mid/.venv/bin/python -m pytest policyengine_us_data/tests/test_database_build.py -k 'national_targets_loaded or distinct_reform_ids' -q