It's turning itself off during boot. Hardware utilities don't detect anything. I tested swapping RAM since that was easy, yielding nothing.
I burned lubuntu to a thumb drive and let it boot; it freezes.
Turned off quiet splash to identify where in the process it freezes. It's here:
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fb: switching to radeondrmfb from EFI VGA.
It has apparently been very hot lately, maybe attributed to playing grim fandango which may or may not run at too high fps these days and cause laptop heat trouble.