Consistent variable suffix to indicate individual: `_indiv`, `_indv`, `_person`

Most tax variables are tax unit-level, but some we have to compute for individuals, e.g. in states that allow married filing separately on same return. We have multiple suffixes to indicate this: `_indiv`, `_indv`, and `_person`. Let's consolidate them. I'd vote for `_person` to avoid abbreviating and tie it to an entity, but welcome suggestions.

Start by locating tax variables that use the `_indiv`, `_indv`, and `_person` suffixes and inventory their definitions and references. Review the naming implications before choosing a convention, then verify that all affected variables and references use the agreed suffix consistently.

Written by the indexing model from the issue text.