can you be my collective memory for a minute, I remember the existence of a very satisfying engineering explanation for why the representation of various body parts needs to be flipped left/right in the brain that came down to the topology of the wiring, and explained why unflipped isn't feasible / or perhaps it was just less efficient, it was one of those 'mind explodes' moments, but now I can't recall the logic.

I recall this. The original idea is from decades ago but perhaps you read it in the Quanta article here https://www.quantamagazine.org/why-the-brains-connections-to... or perhaps that will lead you to what you want.

Are you referring maybe to the "Visual map theory"?

https://en.wikipedia.org/wiki/Contralateral_brain#Visual_map...

I have a similar dim memory, but (at least according to this article) invertebrate bilaterians don't have that swap at all, so it can't be too strong a constraint.

I always thought it was so if an organism takes head damage on one side, the limbs facing the danger will have a better chance to still work, giving it a better chance to fend whatever off and survive.