figured it out, leaving the post up for reference…
The spindle current limit is defined in the machine settings as $800 (part of what makes gCarvin different than plain grbl). I was mucking about and must have set it to zero by mistake. Makes sense that it would try to spin up, sense any amount of current on the pin, and shut down/throw the error. I set it back to 1.75 and I’m back up and running.
Thanks to @NeilFerreri1 and Giovanna at support for pointing me in the right direction!