Add national IRS SOI source packages

[MaxGhenis]

Summary

• add national IRS SOI source packages for tables 1.2, 2.1, 2.5, 4.3, W-2 statistics, and traditional/Roth IRA contributions
• include only the compact source artifacts needed by those packages, avoiding the broader raw-source dump from the umbrella branch
• expand the default Arch bundle from 2 to 9 SOI packages and test the expected semantic duplicate warning across overlapping SOI totals

Verification

• uv run ruff check arch db tests
• uv run pytest tests/test_arch_bundle.py tests/test_arch_source_package.py tests/test_arch_suite.py -q
• uv run pytest -q (610 passed, 16 skipped)
• uv build --wheel --out-dir /tmp/arch-soi-dist plus wheel package-data inspection

Notes

This is the next split from draft PR #3 after the core harness merged in #7. State and congressional-district SOI packages are intentionally left for a follow-up PR because they affect local/geography coverage and should stay separately reviewable.