I'd be activated and angry, too, if I found out that the pwBPD in my life (my husband's kids' mom) were going to work right next to me. Distraction and anxiety would be an understatement

Last year I did worry that she and her husband (uNPD) might show up at my work; I'd had to call CPS about some things the kids told us were going on at their house.
I emailed my manager, my supervisor, and HR to give them a heads up that while I didn't know exactly what would happen or when, they needed to know there was an active CPS investigation and it was possible that the kids' mom & stepdad might contact my workplace by phone, email, or in person. Email meant there was a timestamped "paper trail" of my concern.
Getting ahead of the curve could be smart for you. I wonder if you can email HR and cc anyone relevant in management, stating your concern and suggesting ways to solve the problem. Having it "on the record" before anything happens would seem to go in your favor.
You could play it "low key" at first:
"Hi Manger & HR;
Thanks for working on logistics for RTO. I'm looking forward to collaborating in person again.
You may already know that Ms. Ex and I used to work next to each other, and had been in a relationship that ended on Month/Day/Year. Due to incidents within that relationship, it would be better for productivity and efficiency for us to work [at least X distance apart, on separate floors, in separate buildings, in different departments...].
Please reach out any time if you require supporting documentation such as legal paperwork or police reports, or contact with professionals such as psychologists. If I don't hear back from you by end of day Friday, I'll reach out to ensure we have a workable plan going forward.
Best;
Me88"
That is a fairly neutral, businesslike email, that stays out of the blame game and focuses on solving the problem. I purposefully didn't write it as "I can't work next to her because______" because that can sound like you're the problem, like you aren't allowed to be near her. It also hints that this isn't just you "not liking an ex"; there is a lot more there (documentation, legal stuff, psychologist, etc). And it has a due date in there so you aren't left hanging.
If you get a cooperative response from that, then great. If not, then you could step it up to factually stating why it's a bad idea to have her based near you.
...
It's time to actually grow a backbone and enforce my boundaries. This isn't 'be an adult' or 'just be professional'. I cannot imagine if things were switched, they'd make a woman work near her prior abuser.
Given space limitations I imagine she might somehow end up here again. I'm so angry suddenly.
Sounds like you are activating to take care of yourself. That can be a good thing.