Fix SSI spousal deeming for in-kind support and for aged, blind or disabled spouses who are not eligible #10243

What

Two federal SSI defects in couple and spousal-deeming cases. Both came up while answering the review of PR 10165 (Oklahoma SSP).

1. Spousal deeming uses the $20 general exclusion twice when the eligible spouse has in-kind support

ssi_income_deemed_from_ineligible_spouse measures the deemed amount as couple countable income minus the eligible spouse's countable income alone. Neither figure includes in-kind support and maintenance (ISM). ssi_countable_income then adds the ISM to the eligible spouse's own countable income, which takes its own $20 exclusion.

The deeming steps in 20 CFR 416.1163 combine the eligible spouse's income with the ineligible spouse's deemed income and apply the $20 exclusion once.

Probe (2026-01, Oklahoma):

• Household: aged claimant who receives outside shelter support (PMV 258.50) and a 62-year-old spouse with $700 a month of Social Security.
• Model: countable income is 918.50 (238.50 + 680), so SSI is 572.50.
• One exclusion: 258.50 + 700 − 20 = 938.50, so SSI would be 552.50.

2. The couple classification ignores whether the aged, blind or disabled spouse is eligible

ssi_claim_is_joint, is_ssi_eligible_spouse and is_ssi_ineligible_spouse look only at aged, blind or disabled status. A spouse who doesn't take up SSI, or is barred for immigration status, still gets a couple computation. 20 CFR 416.1160(d) and 416.1801(c) define an ineligible spouse as one who is not eligible for SSI, and deeming then applies (POMS SI 00501.154).

Probe (2026-01):

• Household: aged claimant with $500 a month of Social Security, and an aged spouse with $300 a month who doesn't take up SSI.
• Model: a couple computation, (500 + 300 − 20) / 2 = 390, so the claimant's SSI is 745.50 − 390 = 355.50.
• Under deeming: the spouse's $300 is at most the $497 allocation, so nothing is deemed and SSI would be 994 − 480 = 514.