DoorDash spent 13 years fighting for a spot on your home screen and built an ad business inside that app that's now past a $1 billion annual run rate. This week its cofounder announced a feature whose entire point is that you stop opening the app. Merchants pay those ad dollars to catch your eye while you scroll. Sponsored listings, the stuff you add on impulse. A text thread kills all of it. "Order my usual" produces one order, zero browsing, zero impressions sold. Torching your best margin…