dependent_gross_income counts total Social Security and nets capital losses in the IRC 152(d)(1)(B) test #9678

Problem

dependent_gross_income (policyengine_us/variables/gov/irs/income/dependent_gross_income.py) feeds the qualifying-relative gross income test in is_qualifying_relative_dependent (IRC 152(d)(1)(B)). That test, in turn, drives head-of-household eligibility and filing status. The variable differs from IRC 61 gross income in two ways:

1. Social Security. It replaces taxable_social_security with total social_security (the CIRCULAR_SOURCES map). Only the IRC 86 taxable part is gross income, and a dependent whose provisional income is below the base amount has none.
2. Capital losses. It floors the netted capital_gains source. Gross income includes gains but not losses, so long-term and short-term gains should each be floored at zero before they are added.

Evidence

• IRS Pub. 501 (2022), p. 2: no Social Security is gross income "unless ... one-half of your social security benefits plus your other gross income and any tax-exempt interest is more than $25,000", and "Gross income includes gains, but not losses".
• Pub. 501 (2022), p. 19: the qualifying-relative gross income test counts "taxable social security benefits".
• 26 USC 86 and 26 USC 152(d)(1)(B).
• Example (2022, model-verified while fixing Fix AR, MT and NM income tax gaps found in TAXSIM comparison sweep #9585): the tax-unit head has $50,000 of wages and a dependent parent aged 70 with $12,000 of Social Security and no other income. Provisional income is 0.5 × 12,000 = $6,000, below $25,000, so the parent's gross income is $0, below the $4,400 limit. The parent is therefore a qualifying relative, and the model would then file the head as head of household, as it does when the same parent has $3,000 of unemployment compensation instead. The model counts all $12,000, fails the parent, and files the head as single.
• The capital-loss netting affects results only when the netted amount is below the limit and the gross amount is above it. For example, $5,000 of long-term gains and a $2,000 short-term loss net to $3,000 but give $5,000 of gross income. This example is illustrative and was not run.

Proposed fix

Mirror the Montana measure added in #9585 (mt_dependent_gross_income):

• Split the capital_gains source into long_term_capital_gains and short_term_capital_gains, each floored at zero.
• Replace total Social Security with the IRC 86 taxable part, computed on the dependent's own income. Use provisional income = other gross income + unemployment compensation + tax-exempt interest + combined_income_ss_fraction × benefits, with the single base amounts. This avoids the filing_status cycle that CIRCULAR_SOURCES works around.

After that, mt_dependent_gross_income could reuse the federal measure, excluding only unemployment compensation. Expect changes in head-of-household and qualifying-relative test outputs, and re-check partner contract tests before changing any expected values.

Found while fixing #9585.

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