The Intel 5 SSD holding Win10 system drive on my gaming box went bad at the hardware level, apparently.
I discovered the symptom two weeks back during the 1709 update when it failed to complete and failed to roll back, then dug up enough evidence to reach that fault conclusion after a few reluctant investigation sessions.
Yesterday I caved and bought a replacement SSD of sufficiently antique vintage for this machine, again reluctantly. I don't want to install Windows again.
😣