There are penthouses with great views — and then there’s PH6/7 at 611 West 56th Street, a 5,100-square-foot duplex perched high above Manhattan with its own 1,572-square-foot wraparound terrace. Listed for $15 million, the oversized two-level residence sits in a Pritzker Prize–winner–designed tower and delivers a true bird’s-eye look at the Hudson River, Midtown, and the far west...