systemd's randomized device names changed, so enp0s3 no longer matched enp0s4, and the network didn't come up. It took me a bit to notice, hence the five days. I don't even know if it's a problem with a new kernel or if the provider's virtio_net configuration changed, because journalctl basically locks up when trying to look three or more boots back. Alas!
It took so long to notice because of spam, more or less. I normally look at mail on my phone and respond (if not urgent) from the desktop, but under siege by a 1:40 ham-to-spam ratio, I quit looking at my phone. Unfortunately, I'm also out of the habit of opening Thunderbird unprompted on the desktop.
