Implement South Carolina SSI State Supplement

[MaxGhenis]

Summary

Implements the South Carolina SSI State Supplement (Optional State Supplementation / OSS) program. Replaces #7344 (rebased from upstream/main to resolve conflicts).

Closes #7343

SC's OSS program supplements the income of eligible individuals residing in licensed Community Residential Care Facilities (CRCFs). The program:

• Is available only to individuals in CRCFs (not community residents or nursing home residents)
• Covers both SSI recipients and non-SSI individuals who meet SSA criteria for aged/blind/disabled
• Provides substantial facility payment support (up to $1,719/month to the facility as of 2026)
• Is state-administered by SCDHHS (not federally administered by SSA)

Regulatory authority

• S.C. Code Regs. 126-910 -- Program Definitions
• S.C. Code Regs. 126-920 -- Eligibility
• SCDHHS MB# 26-001 (Jan 8, 2026) -- Current payment amounts

Benefit calculation

OSS = max(0, NIL - ssi_countable_income - ssi)

Files added/modified

• 4 parameter files (net income limit, max facility payment, PNA tiers)
• 3 variable files (eligibility, benefit amount, PNA)
• 1 new input variable (is_in_residential_care_facility)
• 4 test files (31 test cases)
• Modified spm_unit_benefits.py and household_state_benefits.yaml

Test plan

• All YAML tests written with step-by-step calculation comments
• make format passes
• CI passes on GitHub

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (2dd3b64) to head (a75fc1a).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7455      +/-   ##
===========================================
- Coverage   100.00%   96.92%   -3.08%     
===========================================
  Files            2        5       +3     
  Lines           24       65      +41     
  Branches         0        2       +2     
===========================================
+ Hits            24       63      +39     
- Misses           0        1       +1     
- Partials         0        1       +1     

Flag	Coverage Δ
unittests	96.92% <100.00%> (-3.08%)	⬇️

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