Add JCT tax expenditure source package

[MaxGhenis]

Summary

• add a JCT 2024 tax expenditure source package for selected individual income tax revenue-loss estimates from JCX-48-24 Table 1
• include checked-in source rows, manifest provenance, source package metadata, alias, and bundle/source-package tests
• preserve source lineage so Populace can target these facts through simple reform deltas rather than storing target values locally

Source

• JCT publication page: https://www.jct.gov/publications/2024/jcx-48-24/
• JCT PDF: https://www.jct.gov/getattachment/765709fb-9a4b-430a-8f9e-4d342ec97f7e/x-48-24.pdf

Validation

• uv run ruff check arch/source_package.py tests/test_arch_source_package.py tests/test_arch_bundle.py
• uv run pytest tests/test_arch_source_package.py tests/test_arch_bundle.py -q -k "jct_tax_expenditure or jct_tax_expenditure_source"
• git diff --check