Illinois HBWD premium: encode the 21 x 21 premium table instead of adding earned and unearned components #10471

Problem

il_hbwd_premium adds two one-dimensional scales: an earned-income component (gov.states.il.hfs.hbwd.premium.earned) and an unearned-income component (.unearned). The premium table in 89 Ill. Adm. Code 120.510(n)(3) and the HFS HBWD Premium Chart is a 21 × 21 grid: rows are countable earned income bands (0-250, 251-500, … 4751-5000, 5000+) and columns are gross unearned income bands. The grid is not additive.

The scales take the first column (earned) and the first row (unearned). Their sum reproduces 435 of the 441 chart cells and 434 of the 441 regulation cells. Six cells are $1 off in both the regulation and the chart:

Example: $600 of countable earned income and $300 of gross unearned income a month is $31 in the table and $32 in the model.

The seventh regulation cell, earned 1251-1500 with unearned 501-750, is 79 in the regulation and 69 in the chart; the model gives 69 (31 + 38). That is a source conflict, recorded as candidate us-il:hbwd-premium-earned-1251-unearned-501-cell in source_conflicts/candidates.yaml (#10372); the encoded table should follow its ruling.

Proposed fix

Follow policyengine_us/variables/gov/states/ar/ee/aeo/liheap/payment/ar_liheap_matrix_amount.py:

• two band-index scales (earned and unearned) keyed at the band starts that Key Illinois HBWD premium bands at the chart's band starts #10372 introduces (0, 251, 501, … 4,751, 5,001), each returning a band index 0-20;
• one 21 × 21 premium matrix parameter, p.matrix[earned_band][unearned_band], transcribed from 120.510(n)(3) and checked cell by cell against the HFS chart;
• YAML cases for each of the six cells above and the corner cells (both $0, both above $5,000).

Related

• Illinois HBWD premium bands charge the next band's premium at each band edge #10369 (band edges) and Key Illinois HBWD premium bands at the chart's band starts #10372 (its fix, which keeps the additive structure).
• Model working-disabled Medicaid buy-in premiums with state-specific schedules #8513 (state-specific working-disabled buy-in premium schedules).
• Axiom parity: Encode the Illinois HBWD monthly premium tables (89 Ill. Adm. Code 120.510(n)) (PE-US PR 10372) TheAxiomFoundation/rulespec-us#1926, interpretation 3.