No such luck that time;
No such luck that time; navigator link page names are root-relative, and I was missing the ‘/’. It turns out that you can actually learn something from re-reading the docs.
In the process, I got Radio to crash. It seems the networking code isn’t very well threaded — it’d blocked while uploading a file and failed to serve pages while it was blocked. One force-quit and relaunch later and it’s happy again.