QUINCY — It was just a scrap of paper on the floor of a Stop & Shop supermarket in Quincy, near the produce section. But for Marty, no matter how small the scrap, it was one piece too many. At the sight of it, Marty froze and called for backup. Marty is not some teenager working an after-school shift. It is 140 pounds of plastic and metal, with glowing lights atop a towering frame with big cartoon eyes, and cameras and lasers to spot garbage, spills, and other stuff that shouldn’t be in the aisles of a supermarket. The $35,000 machine is one of about 500 robots that Stop & Shop’s owner, the Dutch company Ahold Delhaize, has deployed in some of its US grocery stores. And in the process, Ahold is doing its part to normalize robots in public places.