sfc /scannow
45 minutes later: "Verification 45% complete."
Ah, just as I suspected. There are too many files in this operating system.
It did a bunch of stuff, evidently (at least, the log says so), but none of it fixed the persistently stuck update problem. Also the Windows Update "troubleshooter" and the list of commands that can allegedly be used to erase WU cache.
Anyway, back in comfy Linux land now. Hopefully, next week is the last time I'll need to boot into Windows for the Long Run, at least.
also lol at "CBS.log", not 'cause of the American TV network, but also if you say "C BS en maudit" in French, it pretty much sums up my Windows experience.