Bug#862521: further investigation
Correction: the Realtek NIC doesn't, in fact, require that firmware.
System works the same (aside from the boot message abouit not finding
the firmware file) without it, devices renamed the same... And I
booted into d-i far enough to see that, yep, it sees the new name when
it sets up the NIC.
So I went and did a non-desktop install, and lo, without
NetworkManager installed, it does work as it used to. It's been long
enough, I guess, since I did a new install with a desktop that I'd
forgotten the glitches that brings.
I still think the "improved" NIC naming is a pointless step backwards
for those of us not blessed with 64-NIC firebreathing enterprise NUMA
full-rack machines. Not that I'd want to have that power consumption
and heat generation around here anyway!
BTW, trying to reboot into the previous install (that led to this
report in the first broken place) fails: it hangs with a cursor on a
black screen. So apparently the grub setup for another partition is
messed up. If I decide to put any more effort into this, which at the
moment seems unlikely as I've spent too much time already, I'll
Distributionis non sunt multiplicanda praeter necessitatem.
(William of Occam, in the parallel universe where he was a Linux sysop)