Got Thunderbolt Migration Assistant working!

Had to do half of this: <https://discussions.apple.com/thread/253658404>

- connect Thunderbolt cable to both

On source Mac:  
- create Thunderbolt Bridge interface  
- assign it a 10.0.0.x IP  
- enable Internet Sharing from Wi-Fi to the Thunderbolt Bridge  
- run Migration Assistant

On destination Mac, never set up Wi-Fi. It’ll activate over that Thunderbolt shared-internet connection and never ask for a Wi-Fi network.

(This used to be as easy as “connect them with a cable”.)