Thanks, Walter — that’s got it.
Odd that the 3D Tracker Camera was not active when it was added, but in discovering another workaround just now, I think I figured out why.
I had loaded the clip, time-reversed it, applied the tracker, then added text and camera. When that didn’t work, I tried to take out the time-reversing step to see if that was the problem. I rendered out a separate movie of the reversed clip and tracked that instead — and everything worked fine. So it seems that when you track a time-reversed clip, there’s a bug in the system that turns the 3D tracker camera INactive when it gets added. This situation can be:
a) avoided by tracking a regular forward-playing clip; or
b) fixed using your clear instructions
Many thanks.