NIIT source list omits passive partnership/S-corp and estate income (section 1411(c)(1)(A)(ii))

Found while preparing the business-income crosswalk memo for the TAXSIM collaboration (Microcosm #530 arc), verified against the formulas on current main.

`net_investment_income` adds the parameterized source list `gov.irs.investment.income.sources`: taxable_interest_income, dividend_income, rental_income, loss_limited_net_capital_gains, non_sch_d_capital_gains. Rental income is therefore the only business-income input in the NIIT base.

Per section 1411(c)(1)(A)(ii) and Form 8960 line 4a-4b, net investment income also includes income from passive trades or businesses — passive partnership/S-corp interests and estate/trust income retaining that character. The engine has no passive/active split on `partnership_income`/`s_corp_income`/`estate_income` today, so a full fix needs either a passivity input or a documented assumption; either way the current silent exclusion understates the NIIT base for pass-through-heavy filers.

Flagged in the crosswalk memo to Sabelhaus/Feenberg as an engine-scope note ('we flag rather than paper over it').

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start at net_investment_income and the gov.irs.investment.income.sources parameter, then inspect how partnership_income, s_corp_income, and estate_income are represented. Compare the current source list with section 1411(c)(1)(A)(ii) and Form 8960 lines 4a-4b; done requires either a passivity input or a documented assumption that avoids silently excluding these amounts.

Written by the indexing model from the issue text.