Pin New York itemized deductions to pre-TCJA rules

[DTrim99]

Summary

Closes #6202

NY Tax Law § 615 requires itemized deductions to be computed using federal rules as they existed immediately prior to TCJA (Public Law 115-97).

Changes

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

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 (d08239b) to head (e31bda6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            1         4    +3     
  Lines           10        55   +45     
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+ Hits            10        55   +45     

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

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