Fix SC SALT addback to use pre-OBBBA cap (SC IL #26-4)

[PavelMakarchuk]

Summary

South Carolina has not conformed to OBBBA's 2025 federal SALT cap increase from $10,000 to $40,000 ($20,000 MFS) per SC Information Letter #26-4 Revised, Item 6. The SC1040 line 1d State Tax Addback worksheet must continue to use the pre-OBBBA $10K / $5K MFS cap.

This is a follow-up to the comment on PR #7870.

Bug

sc_state_tax_addback.py reads the federal SALT cap directly:

cap = parameters(period).gov.irs.deductions.itemized.salt_and_real_estate.cap[filing_status]

The federal parameter steps up to $40K at 2025-01-01, so SC inherits the post-OBBBA cap. For 2025+ itemizers, this overstates the SC State Tax addback (worksheet line 5 = cap − real_estate_taxes; a higher cap leaves more room for state-income-tax addback) and therefore overstates SC tax liability.

Direction-of-bias note: the original review on PR #7870 said the bug "understates" SC tax. After tracing the worksheet math, the bug actually overstates SC tax — a higher federal cap means a larger amount of state income tax was deducted federally, which means a larger required SC addback. Either way, the fix is the same: use the SC-frozen $10K / $5K MFS cap.

Fix

1. Add policyengine_us/parameters/gov/states/sc/tax/income/additions/state_tax_addback/salt_cap.yaml holding $10,000 / $5,000 MFS effective 2018-01-01 (the federal TCJA SALT-cap effective date), referenced to SC IL make a test for max snap benefit, add to existing tests #26-4 and the SC1040 worksheet instructions.
2. Update sc_state_tax_addback.py to read this SC parameter instead of gov.irs.deductions.itemized.salt_and_real_estate.cap.

Tests

• Existing 6 pre-OBBBA cases (period 2021): pass unchanged — the SC-frozen $10K matches the then-applicable federal $10K.
• 4 new 2025 cases:
  • MFJ itemizer with SALT just above $10K — addback locked by SC $10K cap
  • MFJ high-SALT itemizer ($30K state income, $10K real estate) — addback drops to $0 (full $10K cap consumed by real estate); under federal $40K cap this would have been $30K
  • SINGLE itemizer with SALT > $10K
  • MFS itemizer — SC $5K cap, not federal $20K

Verification

• 10 / 10 sc_state_tax_addback.yaml tests pass
• 309 / 309 SC tax tests pass
• `make format` clean
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (c271c7a).
⚠️ Report is 136 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8240    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12         1    -11     
  Lines          173        23   -150     
  Branches         4         0     -4     
==========================================
- Hits           173        23   -150     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[PavelMakarchuk]

Closing per maintainer direction.