A happy ending: Despite my pessimism, I was able to recover the images from my card. I'm a bit shocked, actually since I had reformatted the card ...
Interestingly enough, the first few images looked great but then they were clearly just thumbnails and therefore useless.
The problem came up because the connection between my XQD card reader and the computer was accidentally severed, but all the photos seemed to be imported ... I didn't notice the missing master message ... so if that happens to you, be sure to re-import the pictures ...
Edit: I should probably mention that I used PhotoRescue - it worked great. They deserved their $29 and I'm happy .
Right. These days, Aperture actually imports the images to the library, but initially references them on the card itself. This is so the images are usable and editable much more quickly, as you don't have to wait for the image copy to occur before you start editing photos (either to make selections, or to actually make visual changes to them). Once this initial (very fast!) import completes, the masters are copied to your hard drive. Once that is done, you get the notification that the import has completed, and the little spinner stops and the activity window shows as empty.
So be aware of this when you import next time. It's a big win in speed, but you have to leave the card in there so the copy can complete.