After years of watching rivals navigate the uncertain terrain of foldable smartphones, Apple stepped forward on Tuesday in Cupertino with the iPhone Duo — a $1,999 device that signals the company's belief that the technology has matured enough to carry its name. The announcement is less about a single product than about a threshold: when Apple enters a category, it tends to redefine what consumers believe is possible and necessary. The question now is whether the foldable phone, long a curiosity for the few, is ready to become a conviction for the many.