Pin New York itemized deductions to pre-TCJA rules

[DTrim99]

Summary

Implements New York's requirement to use pre-TCJA (Tax Cuts and Jobs Act) itemized deduction rules for state tax purposes.

Per NY Tax Law Section 615, New York itemized deductions must be calculated using federal deduction rules "as such deductions existed immediately prior to the enactment of Public Law 115-97" (TCJA).

Closes #6202

Status

• Documentation collected
• Parameters created
• Variables implemented
• Tests written
• CI passing

References

• NY Tax Law Section 615

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This is a draft PR. Implementation work is in progress.

[DTrim99]

Implementation Complete

This PR implements NY pre-TCJA itemized deduction conformity per Tax Law § 615.

Changes Made

New NY-specific variables (in pre_tcja/ folder):

1. ny_salt_deduction - Uncapped SALT deduction that:

   • Includes state and local sales taxes
   • Includes real estate taxes
   • Excludes state/local income taxes (per § 615(c) add-back requirement)
   • No $10,000 cap

2. ny_misc_deduction - Miscellaneous deduction that:

   • Still applies with 2% AGI floor (not suspended like federal post-2017)
   • Includes unreimbursed employee expenses and tax prep fees

3. ny_casualty_loss_deduction - Casualty loss deduction that:

   • Still applies with 10% AGI floor
   • Not limited to federally declared disasters (unlike federal post-2017)

Updated ny_itemized_deductions_max:

• Now uses NY-specific pre-TCJA components instead of federal itemized_deductions_less_salt
• Adds comprehensive documentation explaining the pre-TCJA conformity

Test Coverage

Added 6 test cases verifying:

• NY SALT excludes income tax, uses sales tax instead
• NY SALT is uncapped (no $10,000 limit)
• NY misc deduction still applies with 2% floor
• NY misc deduction is zero when below floor
• NY casualty loss deduction still applies
• NY itemized deductions max includes all pre-TCJA components

What's NOT Included (per user request)

• Union dues deduction (no input variable in the model)
• Mortgage interest cap changes (input variable is already uncapped)

References

• NY Tax Law § 615
• NY IT-196 Instructions
• NY Itemized Deductions Page

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (062c131) to head (210ad99).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

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- Hits           137        55   -82     

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

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