[codex] Refresh SOI table targets to TY2023

[MaxGhenis]

What changed

• refreshed the tracked SOI table targets in policyengine_us_data/storage/calibration_targets/soi_targets.csv to include TY2023 rows from the IRS workbook release published on March 26, 2026
• added refresh_soi_table_targets.py so future workbook-backed SOI refreshes are reproducible instead of manual
• added focused tests for the non-trivial aggregation logic in Table 1.4 and the interval differencing logic in Table 4.3
• added a make refresh-soi-targets entry point and documented the refresh path

Why

The repo was still carrying tracked SOI table targets through TY2021. IRS published the TY2023 individual workbook tables on March 26, 2026, so this updates those targets and makes the refresh workflow maintainable.

Impact

• calibration code that consumes soi_targets.csv can now use TY2023 workbook-backed targets
• future SOI table refreshes should be a rerun of the script rather than a hand-edited CSV update
• this PR does not update the separate state/district AGI SOI pulls, because the corresponding IRS in54, in55cm, and incd 2023 files are not yet available in the IRS directory

Validation

• regenerated the existing TY2021 workbook-backed rows and confirmed they round-trip exactly before generating TY2023
• pytest -q --noconftest tests/test_refresh_soi_table_targets.py
• python3 -m py_compile policyengine_us_data/storage/calibration_targets/refresh_soi_table_targets.py

[MaxGhenis]

Closing this fork-based PR in favor of #660, which is opened from the upstream branch so full CI can run.