I'm equally mystified, @dropbear42 !
I chrooted into the root partition and found that linux 6.1.arch1-1 was installed, but it only owns files under /usr/lib/modules.
So I re-ran `pacman -S linux`, which re-created vmlinuz and initramfs in /boot.
Not sure where these scripts are. They don't seem to be part of the linux package.