In case anyone else runs into this, I figured out the failure cause.
It's odd -- an unused directory and a file within it each contained the • character. Both needed to be renamed to not include that character. Was never an issue before.
Neither this directory nor anything in it is referenced by the Shiny app, and I don't think peeky::peek_standalone_shinylive_app() reveals that directory or its contents, but apparently shinylive::export() now notices and fails if the offending filenames exist.
Not closing this as "Solved" in case anyone more familiar with shinylive::export() can weigh in on whether this is expected behavior.