Thank-you for the suggestion, Wolfgang.
I tried "journalctl -n 100000" as root.
I did find "boot --", but I saw no hint of the error messages.
I searched for "grub". I searched for "error". No hits.
This is a dual-boot workstation. The error messages show up *before*
the grub menu appears, so no operating system would yet have been
chosen. So if grub logs error messages anywhere, wouldn't they be
somewhere independent of operating system? But then, operating systems
and boot processes are black boxes to me.