Clean up reform validation cache and add regression tests

[MaxGhenis]

Summary

Follow-up to merged PR #639.

This cleanup PR addresses a few non-blocking review items from the JCT/SOI target fix:

• make the validation helper cache the final reform income_tax delta, rather than briefly caching raw reform values and then overwriting them in the caller
• add a unit test covering the reform-delta cache contract in validate_staging.py
• add an ETL regression test covering stale baseline-row deactivation and rerun behavior for national tax expenditure targets

Why

PR #639 fixed the economics and the calibration path. This PR just tightens the maintenance story around that fix and adds regression coverage for two of the follow-up risks reviewers flagged.

Validation

• python3 -m py_compile policyengine_us_data/calibration/validate_staging.py policyengine_us_data/tests/test_calibration/test_validate_staging.py policyengine_us_data/tests/test_etl_national_targets.py
• ruff format on the touched files

Environment limitation

I did not run the full pytest path locally in this workspace because the available local environments are still broken on torch / missing sqlmodel.