Does it ever explicitly say in Skyrim, or in ANY lore for that matter, that a "Dragonborn" has the SOUL of a dragon? I know Todd Howard said it but maybe ive just missed the textual proof... is there any?

no one ?

This is a fair point.

It's not outright stated, but implied. Of course, I can live with dragonborns not having a dragon soul, because it makes it easier for Skyrim's PC to be a Shezzarine.

Martin Septim broke the amulet of kings, and absorbed dozens of the Dragon Souls of previous emperors, becoming an aspect of Akatosh.

Paarthurnax speaking to you as if you were a fellow Dovah is fairly conclusive to this. I doubt he would regard anyone else (greybeards included) in this manner.

The Dovahkiin is literally just a dragon in a mortal body.