Add SNAP immigration status eligibility

[hua7450]

Summary

Closes #1475

Adds SNAP immigration status eligibility, reflecting changes from the One Big Beautiful Bill Act of 2025 (OBBBA). Excludes immigration-ineligible members from SNAP unit size for correct benefit calculation in mixed-status households. Implements California's delayed effective date (April 1, 2026) per ACL 25-92.

Regulatory Authority

• 7 USC 2015(f) - Eligibility disqualifications
• 8 USC 1612 - Limited eligibility of qualified aliens for certain Federal programs
• USDA FNS - SNAP Implementation of OBBB - Alien SNAP Eligibility guidance
• 7 CFR 273.11(c) - Treatment of income and resources of excluded household members
• California CDSS ACL 25-92 - CalFresh implementation of H.R. 1 noncitizen eligibility changes

Changes

Parameters

• gov/usda/snap/eligibility/eligible_immigration_statuses.yaml
  • Pre-OBBBA (2018-01-01): CITIZEN, LPR, REFUGEE, ASYLEE, DEPORTATION_WITHHELD, CUBAN_HAITIAN_ENTRANT, CONDITIONAL_ENTRANT, PAROLED_ONE_YEAR
  • Post-OBBBA (2025-07-04): CITIZEN, LPR, CUBAN_HAITIAN_ENTRANT only
• gov/states/ca/cdss/snap/eligibility/eligible_immigration_statuses.yaml
  • Same pre-OBBBA list, but restricted list effective 2026-04-01 (California delayed implementation per ACL 25-92)

Variables

• is_snap_immigration_status_eligible - Person-level variable checking if immigration status is eligible; uses federal_eligible | ca_eligible to apply CA's delayed date
• ca_snap_immigration_status_eligible - California-specific immigration eligibility with defined_for = StateCode.CA
• Updated is_snap_eligible to require at least one household member with eligible immigration status
• Updated snap_unit_size (renamed from snap_unit.py) to exclude immigration-ineligible members, using OR with ineligible students to prevent double-counting

Mixed-Status Households

• is_snap_eligible: Uses spm_unit.any() — household is eligible if at least one member has qualifying status (7 CFR 273.11(c))
• snap_unit_size: Subtracts ineligible members so allotment, standard deduction, and min allotment are based on eligible member count only
• Income proration for ineligible aliens is not yet implemented (separate follow-up)

Known Limitations

• COFA citizens: Eligible post-OBBBA per federal law, but COFA is not in the ImmigrationStatus enum. Needs to be added separately.
• LPR 5-year waiting period: Cannot be modeled in PolicyEngine's single-period architecture.
• Income proration: Per 7 CFR 273.11(c), ineligible aliens' income should be prorated. Not yet implemented.

Test Plan

• is_snap_immigration_status_eligible - 10 tests covering federal and CA-specific behavior
• ca_snap_immigration_status_eligible - 8 tests covering CA delayed implementation
• snap_unit_size - 9 tests including mixed-status households and double-counting prevention
• is_snap_eligible - 8 tests including undocumented ineligible student edge case
• Existing SNAP eligibility tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (901599b) to head (cd0f7df).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

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  Files            3         4    +1     
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+ Hits            54        58    +4     

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