After a decade of quiet engineering ambition, Apple steps into the folding future with the iPhone Duo — a $2,000 device that asks whether the next chapter of mobile computing is worth the price of admission. New CEO John Ternus takes the stage Wednesday not merely to announce a product, but to inherit and fulfill a vision that consumed his predecessor's final years. In entering a market Samsung has long owned, Apple is not following a trend so much as declaring, in its characteristically deliberate way, that the trend has finally arrived.