Thanks for the mention. I was getting ready for bed when I noticed you were asking me a question.

I think your best clue at the moment for identifying it is the spiral arrangement of the leaves. See how they climb around the trunk? From the top you see a triangle. Very unique! The serrated leaves are also a good clue, as you point out, yet many trees have this trait.

Yours doesn't seem to resemble the cherry cuttings I grew. Your leaves are too long and narrow. However, my cuttings were from mature tree wood, and so new leaves might look a lot different than those on a brand new seed tree. For instance, Ash trees grow a twin pair of long thin leaves from seed, like a carrot sprout, and then it develops true leaves that are wide and full.

Cherry cutting, new leaves. Rooted in gravel and sand.

Out of the tree seeds I planted in late Winter, only one of them sprouted, and I am also having trouble identifying it. Whatever I planted, it could be a weed or it could be exactly what I planted. Hopefully it is the Siberian Flowering Almond from the seed I planted. It looks a bit similar to your seedling, so I'll have to take a photo tomorrow to show.

My gut says you've got some kind of stone fruit, or nut tree. Many trees with serrated leaves fall into this plant classification, and different stone fruits can even be grafted onto each other.