Italian super-yacht designer Achille Salvagni has dropped anchor on Madison Avenue, The Post has learned. Salvagni just paid $13.5 million for 797 Madison Ave., the former Tory Burch flagship building that last traded nearly a half-century ago. Salvagni, who has a showroom at 817 Madison Ave. a few blocks away, is known for high-end home interiors as well as for $50 million yachts. The deal was broken by Meridian Investment Sales’ Henry Barnathan and David Schechtman.