Iowa and Mississippi move a dependent's net income onto the head's state return #9887

Two state models move a tax-unit dependent's net income onto the head's state return:

• Iowa: policyengine_us/variables/gov/states/ia/tax/income/net_income/ia_net_income.py returns ~is_dependent * net_income + is_head * sum_dep_net_income ("allocate any dependent net_income to tax unit head").
• Mississippi: policyengine_us/variables/gov/states/ms/tax/income/ms_agi.py does the same ("Allocate income from dependents to tax unit head").

Federally, #9801 keeps a dependent's items on the dependent's own return, and #9866 keeps a dependent's business income off the filer's QBI deduction (and Iowa's ia_qbi_deduction). So in Iowa a dependent's business income still reaches the parent's taxable income through ia_net_income, now without any QBI deduction (flagged in #9866's review). #9801 kept the Mississippi transfer and tests it in ms_agi.yaml.

To do: for each state, check the statute and return instructions (Iowa Code ch. 422 and the IA 1040 instructions; Miss. Code and Form 80-105 instructions). Decide whether a dependent's income belongs on the parent's return, for example through an election like federal Form 8814, or on the dependent's own. Then remove the transfer with tests and a microsim, or document the law in the variable.

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