when i reviewed the session i noticed that the following were running just before i rebooted:
389 ?        Ss     0:00 /lib/systemd/systemd-udevd --daemon
1804 ?        S      0:00 /lib/systemd/systemd-logind

i also noticed that libpam-systemd had (of course) been removed by apt-get but didnt think to check what it had been replaced with.

it seems too late to fix, so i will have to ask the remote colo provider to reinstall a fresh copy of debian 9.  in case i decide to try migrating again, would you folks have any hints what i should look for, just prior to the 2nd reboot, to ensure the system will be accessible afterwards?

[update]
when i got the freshly installed machine the ethernet interface was named "eno1" for some reason and i left it as is.  the migration may have automatically renamed it somewhere to eth0 (perhaps in the udev rules?), but not in /etc/network/interfaces .  also something was different about the sshd setup.  the colo providers fixed the issue and i am back up and running!