Construction has surpassed the halfway mark on 520 Fifth Avenue, an 88-story residential supertall skyscraper in Midtown, Manhattan. Designed by Kohn Pedersen Fox and developed by Rabina, the 1,000-foot-tall structure will span 415,000 square feet and yield 100 condominium units, a residents-only amenity floor on level 88, boutique office floors, a private members only club called Moss,…