Apply SSI's couple eligibility test when one spouse is temporarily absent in a facility #10851

What happens

When one spouse of an eligible SSI couple is in a Medicaid facility, ssi_couple_computation_applies stops the couple computation, and the model tests each spouse on their own income alone. SSA first tests the couple's eligibility on their combined income, and only then pays each spouse on their own income.

Example from #10820 and its review, 2026: spouse A at home has $1,000 a month of Social Security, and spouse B, in a facility, has $1,500. The model pays A $994 - $980 = $14 in each facility month, although the couple's $2,480 of countable income is above the $1,491 couple rate. main pays the same $14 when the stay covers December.

What the law says

• 42 U.S.C. 1382(a)(2)(A) tests an individual with an eligible spouse on income "(together with the income of such spouse)".
• 20 CFR 416.1801(c): "An individual is considered to be living with an eligible spouse during temporary absences as defined in § 416.1149".
• POMS SI 02005.050 B.3: "If one member of an eligible couple is temporarily absent as defined in SI 00835.043, they are considered an eligible couple. Determine SSI eligibility by subtracting the couple's combined countable income from the full FBR for a couple. If they are eligible, determine the amount of their SSI payment based on each person's own income."

Why it isn't modeled

The test applies only to a temporary absence, and the model has no input for whether a stay is temporary or whether the spouse intends to return. ssi_couple_computation_applies already notes that intent to return is not modeled.

Fix

Add an input for a temporary absence, and in those months test the couple's combined countable income against the couple rate before paying each spouse on their own income.

Related: #10818.