Is that poly blend? Three raps over the knuckles with a braided belt! Preppy fashion is all about rules—the standard bearers sniff at synthetics, cleavage, sparkles, and a million other don’ts. But those dictates source originally from the “windswept and privileged” attire of 1920s college men, the supposed first to co-opt athletic gear as daywear in an unstudied, thrown-together way. Preppy, released on October 4 from Rizzoli, lays out the evolution of Ivy style in a sleeker, glossier way…