Okay . . . The problem seems to be solved!
What I did:
1) Booted into Debian 11.6 Live/install usb thumb drive.
2) As root, mounted the / partition from the internal ssd.
3) On the internal ssd, replaced /etc/fstab with /etc/fstab.original.
(On the internal ssd, I did not delete /tmp or its contents, and did
not delete the tmp partition or its contents.)
4) Unmounted the / partition on the internal ssd.
5) Shutdown and removed the usb thumb drive.
6) Booted in to the computer as usual.
It *seems* to have worked fine, with /tmp mounted on its dedicated
partition again.
But there may still be leftover stuff in /tmp, so maybe later I will
again boot from the live usb, delete everything in /tmp (but not /tmp
itself) on the internal ssd, and reboot into the system, which will
presumably have re-populated with no leftovers.
So far, so good.
Much thanks to all who have weighed in on this!