Toggle to make UBI state taxable

State-level UBI payments like universal child allowances are likely to be taxed federally but not by the state. #1178 would make them federally taxable. State taxability is trickier because we have different state-level AGI formulas, but adding it would improve realism.

Start by reviewing issue #1178 and the project's state-level AGI formulas, since no files, tests, or entry points are named here. Determine how a toggle should control state taxability across differing state formulas; done means the toggle works consistently for state-level UBI payments and is covered by appropriate validation.

Written by the indexing model from the issue text.