Add MO $1,400 head of household additional exemption (Line 15)

[PavelMakarchuk]

Summary

• Add the $1,400 Missouri additional exemption for Head of Household and Qualifying Surviving Spouse filers (MO-1040 Line 15)
• This is a separate MO-specific provision, not tied to the suspended federal personal/dependent exemptions

Closes #7793.

Change

• New parameter: gov.states.mo.tax.income.exemptions.head_of_household = $1,400
• New variable: mo_head_of_household_exemption — returns $1,400 for HoH/QW filers, $0 otherwise
• Updated mo_taxable_income formula to subtract the exemption alongside other deductions
• Updated existing TAXSIM integration test (HoH taxable income now $1,400 lower, closer to TAXSIM reference)

Test plan

• 5 unit tests for mo_head_of_household_exemption (HoH, QW, Single, Joint, Separate)
• 2 integration tests for mo_taxable_income (HoH with exemption vs Single without)
• Updated TAXSIM integration test reflects corrected values
• All 251 MO tests pass
• make format clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6e42726) to head (1602ecb).
⚠️ Report is 123 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #7824    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           64         2    -62     
  Lines          707        38   -669     
  Branches         1         0     -1     
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- Hits           707        38   -669     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[DTrim99]

PR Review: Add MO $1,400 Head of Household Additional Exemption

🟢 No Critical Issues Found

This PR is well-implemented and ready to merge.

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Validation Summary

Check	Result
Regulatory Accuracy	✅ $1,400 matches RSMo 143.161(2)
Reference Quality	✅ Statute explicitly states "one thousand four hundred dollars"
Code Patterns	✅ No hard-coded values, proper parameter usage
Test Coverage	✅ All 5 filing statuses + integration tests
CI Status	✅ All checks passing

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What Was Verified

Regulatory Research:

• RSMo 143.161(2) confirms: "a resident who qualifies as an unmarried head of household or as a surviving spouse for federal income tax purposes may deduct an additional one thousand four hundred dollars"
• Amount: $1,400 ✅
• Eligibility: HoH and Surviving Spouse ✅
• Effective: Since January 1, 1999 (parameter starts 2021 - conservative but correct)

Code Quality:

• Uses parameter (p.head_of_household) not hard-coded value
• Correct entity level (TaxUnit)
• Proper defined_for = StateCode.MO
• Reference tuple format is correct

Test Coverage:

• HEAD_OF_HOUSEHOLD: $1,400 ✅
• SURVIVING_SPOUSE: $1,400 ✅
• SINGLE: $0 ✅
• JOINT: $0 ✅
• SEPARATE: $0 ✅
• Historical years (2021, 2022, 2024) ✅
• Integration tests with mo_taxable_income ✅

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🟡 Minor Suggestion (Optional)

YAML section ordering: The parameter file has values: after metadata:. The standard pattern places values: before metadata:. This works correctly but could be reordered for consistency:

description: ...
values:
  2021-01-01: 1_400
metadata:
  ...

This is cosmetic and not blocking.

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Recommendation

APPROVE - Clean implementation that correctly reflects Missouri law.

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