That gave me an idea - I copied the boot partition from the (failed) upgrade while leaving the / partition intact. Just in case there were other things in the staging repo which weren’t working. I got
Illegal instruction (probably - it scrolled off the screen)
Failed to start Load Kernel Modules
...
Then it had trouble mounting dev-mmcblk0p1.device (Start job timed out)
Emergency mode, but couldn’t run the suggested journalctl. /boot not mounted.
Warm reboot - a different set of error messages but starting with Illegal instruction and ending with the sad face as the first attempt. /boot is now mounted. Couldn’t upload logs - some message about not being able to get a URL, but I’ve posted them on dropbox.