AK ATAP maximum payment undercounts two-adult assistance units

\`ak_atap_maximum_payment\` reconstructs the payment standard as a base ($821, caretaker + first child) plus $102 per additional **child** (\`gov.states.ak.dpa.atap.payment.base\` / \`.additional_child\`). The DPA ATAP Standards 2026 adult-included chart is keyed by **assistance unit size**: 2 → $821, 3 → $923, 4 → $1,025, 5 → $1,127, … (+$102 per additional person).

The two constructions coincide for single-parent units but diverge whenever the unit includes a second adult.

**Repro** (policyengine-us 1.774.5, period 2026-07): a 5-person spm_unit in AK with two adults (ages 30, 28) and children aged 8, 5, 2 returns \`ak_atap_maximum_payment = 1025\`; the published chart row for size 5 is $1,127.

\`ak_atap_need_standard\` for the same unit is correctly size-keyed (returns $2,629), so only the max-payment parameterization needs the fix.

Found while extending the engine-vs-PolicyEngine oracle suite in TheAxiomFoundation/finbot-snap-demo (PR #21); our independent encoding of the same chart (by family size) is the source of the $1,127 figure.

Locate `ak_atap_maximum_payment` and its `gov.states.ak.dpa.atap.payment.base` and `.additional_child` parameters, then compare their construction with the size-keyed `ak_atap_need_standard`. Reproduce the 2026-07 five-person unit with two adults and three children, and confirm the maximum payment matches the published $1,127 chart row without changing the need standard.

Written by the indexing model from the issue text.