The closet partition stud was bearing enough load to matter, but it wasn't holding up the whole second floor by itself.

I put in a new beam to replace the load bearing wall the preivous-previous owners had removed, with posts down to the basement: https://www.jefftk.com/p/bathroom-construction-framing

The only option is to do a complete rebuild. The load bearing stud indicates that the foundation design was inadequate for current needs.