Mac Photos stuck on "Restoring... 0%" after Time Machine restore
I solved this with the following steps:

Restart in Safe Mode (Shift key held down).
Attached an external drive containing a clone I’d made the same day as the Time Machine backup.  I don’t remember if this clone was made with Super Duper or Carbon Copy Cloner, as I have both.
Copied the Photos Library from the external drive to the internal drive.  I made this copy with the ditto command in the Terminal.
Disconnected the external drive
Launched Photos, and verified the photos (in low-resolution) were present.
In Photos, set that just-copied Photos Library to be Primary.
Reboot normally
Waited about five minutes for all startup routines to complete
Launched Photos and enabled iCloud Photos
Verified I could access high-resolution versions of the photos (confirming iCloud connectivity).
Waited overnight for photos newer than the clone date to appear, which they did.