Add a new variable for adjusted gross income for charitable deductions to align with IRC definition (§ 170(b)(1)(H))

[MaxGhenis]

Current behavior

The positive_agi variable is currently calculated as shown in:

policyengine-us/policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/positive_agi.py

Line 13 in 0a5bb3c

return max_(tax_unit("adjusted_gross_income", period), 0)

This is used in multiple places, including the charitable_deduction formula:

policyengine-us/policyengine_us/variables/gov/irs/income/taxable_income/deductions/itemizing/charitable_deduction.py

Line 18 in 0a5bb3c

positive_agi = tax_unit("positive_agi", period)

However, the Internal Revenue Code (IRC) defines the contribution base in § 170(b)(1)(H) as:

Contribution base means adjusted gross income (AGI), computed without regard to any net operating loss (NOL) carryback to the taxable year (§ 172).

PolicyEngine does not currently distinguish between AGI and the contribution base as defined by the IRC, and no variables exist to represent NOLs or NOL carrybacks.

Problem

Using positive_agi for charitable deductions:

1. Does not exclude NOL carrybacks, potentially overstating allowable deductions for taxpayers with reduced AGI due to NOLs.
2. Impacts other calculations that rely on positive_agi, creating potential inconsistencies if we try to modify positive_agi directly.

Proposed solution

1. Create a new nol_carrybacks input variable
2. Create a new charitable_deduction_contribution_base variable which equals adjusted_gross_income - nol_carrybacks