Add ssi_couple_computation_applies for Title XIX facility separation

[hua7450]

Summary

Adds a new ssi_couple_computation_applies variable that implements 20 CFR 416.414's medical facility separation rules, without overloading ssi_claim_is_joint.

Closes #8003

Why a New Variable (Not Modifying ssi_claim_is_joint)

The issue originally proposed modifying ssi_claim_is_joint directly. However, ssi_claim_is_joint carries two meanings downstream:

1. "Are both spouses SSI-eligible?" — used by state SSP programs (AK, MA, DE, CO) for claim type classification
2. "Should federal SSI use couple computation?" — what 416.414(b)(3) affects

These diverge when one spouse enters a medical facility: both are still SSI-eligible, but federal SSI computes benefits separately. Overloading ssi_claim_is_joint to return false would misclassify state SSP claim types — e.g., Alaska's ak_ssp_claim_type would move a two-eligible-spouse couple onto the COUPLE_ONE_ELIGIBLE payment schedule.

Solution: ssi_claim_is_joint retains "both eligible" semantics. The new ssi_couple_computation_applies encodes the facility separation rule and is used only in federal SSI calculations.

Regulatory Authority

• 20 CFR 416.414(b)(2): Both in facility → couple computation still applies ($60/month = $30 each)
• 20 CFR 416.414(b)(3): One in facility → benefits computed separately; facility spouse gets $30, community spouse gets individual rate per §416.410
• POMS SI 00501.154: Couple computation rules require living in the same household; "sharing a room in an institution is not living in the same household"

What Changed

File	Change
ssi_couple_computation_applies.py (new)	joint_claim & ~one_in_facility — FALSE when exactly one spouse is in a Title XIX facility
ssi_amount_if_eligible.py	Uses new variable for base amount; removed redundant community spouse override
meets_ssi_resource_test.py	Uses new variable for couple vs individual resource limits
ssi_pmv_amount.py	Uses new variable for couple vs individual PMV
ssi_claim_is_joint.py	Unchanged — reverted to original
State SSPs (AK, MA, DE, CO)	Unchanged — still use ssi_claim_is_joint

Test Plan

5 new test cases in ssi_couple_computation_applies.yaml:

Case	Scenario	ssi_claim_is_joint	ssi_couple_computation_applies
1	Eligible couple, neither in facility	[true, true]	[true, true]
2	One spouse enters Medicaid nursing home	[true, true]	[false, false]
3	Both spouses in medical facilities	[true, true]	[true, true]
4	Spouse in privately-funded facility (not Title XIX)	[true, true]	[true, true]
5	Only one spouse eligible, no facility	[false, false]	[false, false]

• All 148 SSI tests pass
• All 48 AK SSP tests pass
• All 21 DE SSP tests pass
• All 13 MA SSP tests pass
• All 8 CO OAP tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.30%. Comparing base (7f3ec13) to head (67136d8).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
...v/ssa/ssi/eligibility/income/ism/ssi_pmv_amount.py	0.00%	1 Missing ⚠️
...ssa/ssi/eligibility/income/ssi_countable_income.py	0.00%	1 Missing ⚠️
...us/variables/gov/ssa/ssi/ssi_amount_if_eligible.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #8004       +/-   ##
============================================
- Coverage   100.00%   65.30%   -34.70%     
============================================
  Files            2        6        +4     
  Lines           46       98       +52     
============================================
+ Hits            46       64       +18     
- Misses           0       34       +34     

Flag	Coverage Δ
unittests	65.30% <84.21%> (-34.70%)	⬇️

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