The new version of elogind seems to have fixed the problem. Thanks to all who answered!

Thanks for the quick reply. FWIW here's the full error message:

~> sudo apt upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libelogind-compat : Conflicts: libsystemd0
E: Broken packages

The "apt upgrade" error persists even after "apt full-upgrade" succeeds, which is part of what confuses me. The other thing that confuses me is that libsystemd0 isn't installed, so can't conflict anything.

I tried "apt purge libsystemd0" in case there's something lurking, but the error remains.

Further update: I noticed that systemd-standalone-tmpfiles is in the autoremove list, so I removed it manually. Also no effect.

For what it's worth - just ran apt-update and noticed new versions of eudev and libeudev1 incoming. After upgrading them (and only them) PC boots with no problems.
Then did a full upgrade. Now cryptsetup is causing trouble, but that's a different matter.

I got the warning from dpkg too, and a few warnings about dependency problems during the upgrade.

I've found a solution or a workaround: downgraded to eudev_3.2.12-4+deb11u1_amd64.deb and libeudev1_3.2.12-4+deb11u1_amd64.deb
Removed my hacked list of modules from /etc/initramfs-tools/modules and ran update-initramfs -u

Now my system boots kernel 6.6.15 correctly. Keyboard and mouse both working. At some point I should tidy up /boot again and regenerate all the initrd.img files

So it might not be related to usrmerge after all.

What now? Should I report a bug against eudev?

First inkling of a problem:  https://bugs.devuan.org/cgi/bugreport.cgi?bug=828

That was before usrmerge was forced. Some scripts had hard-coded paths to /bin and /sbin, but the programs they use moved to /usr/bin and /usr/sbin

But the problem of hard-coded paths should go away after usrmerge, shouldn't it?

An update at the weekend (unstable/ceres) forced a usrmerge. Since then the PC doesn't boot correctly. The error message says that if cannot find the root filesystem.

The immediate cause is that the kernel fails to load any modules. I forced that by adding a list of modules to /etc/initramfs-tools/modules - the list obtained from lsmod in the repair system. Now the PC boots. I get a gui login. Unfortunately X doesn't recognise the keyboard and mouse. I can log in by ssh from my laptop and see that there are no errors in Xorg.log.0, but also no information about loading X modules for HID.

My guess is that the problem is related to udev.  Any ideas or comments?