Apply MT taxable Social Security base amounts once per return (#9011)

[PavelMakarchuk]

Fixes #9011.

Problem

mt_taxable_social_security runs the 2021–2023 Form 2 Taxable Social Security Benefits Schedule per person but took the line 10/12 base amounts by tax-unit filing status. So a married-joint couple got $32,000/$12,000 each ($64,000/$24,000 combined) instead of once, and the married-filing-separately-on-same-form columns never got their $16,000/$6,000 amounts.

2021 Form 2, page 6:

Line 10: "Married filing jointly, enter $32,000 in column A; … Married filing separately, enter $16,000 in columns A and B."

This understated Montana taxable SS for essentially every married unit with benefits in 2021–2023. On taxsim #1078, TaxAct (status 2a columns) reports 34,166 + 20,539 = 54,705; PolicyEngine reported 23,337 + 5,318 = 28,655.

Fix

• The person-level mt_taxable_social_security (used by the separate-on-same-form path, mt_agi_indiv) now maps joint filers to the married-filing-separately half-of-joint base amounts ($16,000/$6,000) so each spouse's column is computed once at the half amount.
• New mt_taxable_social_security_joint (tax-unit) runs the schedule once on combined income with the full joint base amounts ($32,000/$12,000), and mt_agi_joint now consumes it instead of summing the per-person result. This keeps the true single-column joint computation correct.

Tests

• Unit: split the old "filing jointly" case into a per-column case ($850, separate-on-same-form) and a joint single-column case ($500, mt_taxable_social_security_joint) to document both paths.
• Integration: added the taxsim Colorado Works (TANF) #1078 case → mt_taxable_social_security: [34_166, 20_539] (matches TaxAct status 2a columns).
• Full MT suite (536 tests) passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ff39625) to head (b825dab).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

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##              main     #9018   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         3    +2     
  Lines           18        92   +74     
  Branches         0         1    +1     
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+ Hits            18        92   +74     

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unittests	100.00% <100.00%> (ø)

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