After a reboot, I traced it back to a folder in `Library/Application Support` which included a `config.json` that mentioned gulf[.]moneroocean[.]stream, so I knew I'd been popped. Cleanup was easy enough (the files themselves, plus the launch daemon), but it's annoying.
I don't do dev work on that machine (it's really just for plex & stuff) so it must have been some sort of vulnerability in Plex (but not the one described in OP) or macOS itself. Interestingly, I was up to date on my Plex install, but my macOS version was behind what was apparently an important security update related to an RCE (Sequoia 15.8).