I did a bunch of online reading, particularly looking for photos of corydora fry in various stages. When they first hatch, they look like little tadpoles- head and a tail with continuous fin around the edge. Mine already had distinct fins by the time I saw it, so I think it's already over two weeks old.
It was at the base of this wisteria, to the left of the roots. Twice yesterday I saw Momma mouthing it, and bumped the glass there with my finger, to scare her off. But I guess she got it. Or it hatched- timing of that is three to seven days, so it's possible I'll see another fry emerge from the thickets in a week or two... maybe.
I'm still eager to take daily photos of the baby cory, Lucky. Here's the best from past two days. Under a white oak leaf: