Thanks for all the responses!
I think you guys are right.
The stem is much redder than any of the photos of Koelreutaria paniculata I could find and the redness extends into the veins of the leaves, but this could just be a variation.
Messy and edible according to Pfaf.
I think it won't transplant well, having sprouted in a crack of the wall.
I can't let it grow so big that its destroys the wall, but the
roots are supposedly not aggressive.
Koelreutaria paniculata is hermaphroditic so perhaps it will produce viable seed before it gets too big.
https://pfaf.org/user/Plant.aspx?LatinName=Koelreuteria+paniculata