Luxury ­timepieces are back. They’re highly collectible, exceptionally crafted…and costlier than ever.Note the watch on your left. Introduced in January at Geneva’s Salon International de la Haute Horlogerie, the Openworked Extra-Thin Royal Oak Tourbillon in stainless steel from Audemars Piguet embodies several prevailing trends in the luxury watch market. First and foremost? ­Heritage. The Royal Oak turns 40 this year, and Audemars is sparing no expense in feting it. Across the watch business, rare is the debut that does not evoke a past glory—one sign of how important the notion of ­tradition is to the category. Next, consider its skeletonized tourbillon movement. The ­combination of watchmaking’s most conspicuous feature and a see-through display speaks to the rise of the collector class, whose members helped Swiss watch exports reach 19.3 billion francs ($20.9 billion)