The possible primrose (Oenothera sp.) has started to flower, and is confirmed as a primrose. There may be some question about the species still, but O. biennis, the common evening primrose, looks enough like this that to make it the leading candidate as far as I'm concerned.
As to why it grew with multiple shoots, instead of the single upright stem that's (I gather) more typical of the species, I couldn't tell you. There are pretty definitely multiple stems radiating away from a central point, though.
Am I happy about having let it grow? I dunno. It's something new to me, and it was interesting to watch, if nothing else. I'm not sorry I let it stay.
It did attract Japanese beetles, which would matter more to me if I had anything else outside that Japanese beetles seemed to like, but as far as I can tell, there haven't been many and they've only been on the Oenothera. (No doubt the neighbors on the other side of the fence would prefer I not be attracting Japanese beetles.) I'll pull up any seedlings I see next year, but this was an okay experience.