Model state-specific SNAP BBCE poverty-guideline update months

[hua7450]

Problem

The repository already contains the updated 2026 HHS poverty guideline parameters. However, the SNAP BBCE gross-income test currently uses snap_gross_test_income_fpg_ratio, whose snap_fpg denominator changes on October 1 with the federal SNAP fiscal year.

State BBCE schedules may not all follow that October update. Washington is a confirmed example: WAC 388-414-0001(2)(a)(ii) applies the current-year federal poverty guidelines beginning April 1 each year.

As a result, for a one-person Washington assistance unit in 2026:

• January–March: $2,608/month (200% of the 2025 guideline)
• April–December: $2,660/month (200% of the 2026 guideline)

The model currently keeps the $2,608 limit through September, making the Washington cutoff approximately 2% too low from April through September.

Relevant code:

• policyengine_us/variables/gov/hhs/tanf/non_cash/meets_tanf_non_cash_gross_income_test.py
• policyengine_us/variables/gov/usda/snap/income/snap_fpg.py
• policyengine_us/parameters/gov/hhs/tanf/non_cash/income_limit/gross.yaml

Proposed work

• Add a BBCE-specific mechanism for selecting the applicable FPG vintage without changing snap_fpg, which should retain the federal SNAP October schedule.
• Encode Washington's April update schedule.
• Verify the BBCE poverty-guideline effective month for every other BBCE state using state or other authoritative sources, and encode any additional exceptions.
• Document which states have been verified and the supporting sources.
• Add boundary tests immediately before and after each non-October transition, including income at and just above the applicable limit.

Acceptance criteria

• Washington's one-person BBCE limit changes from $2,608 to $2,660 in April 2026, not October 2026.
• Regular federal SNAP income tests continue using the federal October update schedule.
• Other states' BBCE update months are verified and represented accurately.
• Tests cover Washington's March/April boundary and any other confirmed state-specific boundaries.