It's the centerpiece of the largest and most expensive private real estate development in American history: "Vessel," a 150-foot-tall sculpture in the middle of Hudson Yards, a new 16-building complex on the West Side of Manhattan. The honeycombed flight of fancy, made of 154 flights of stairs, was conceived by 49- year-old British designer Thomas Heatherwick, who has put his stamp on some of the most provocative buildings and public projects around the world. Anthony Mason reports.