For the first time ever, the 'Catwalk' book series spotlights the history of an American designer. Of course, it's Ralph Lauren.Credit: Getty Images / InStyleFrom the streets of Sag Harbor to the beaches of Santa Monica, no matter where you are in America, there’s one sartorial guarantee for Memorial Day weekend. Someone’s going to show up to the function in a Ralph Lauren flag sweater.You know the one. It’s been made internet-famous for its eas…