By Manishi RaychaudhuriHONG KONG, Sept 8 – South Korea's brutal summer equity rout showed how margin debt can turn a selloff into a market-wide tailspin. Korea's AI giants remain exposed, but the next Asian crash may follow different fault lines.The sharp correction in South Korean equities, which saw the KOSPI collapse 39% from June 22 to July 30, has been widely attributed to the rapid buildup and subsequent unwinding of margin loans held by retail investors.Driven by AI euphoria, millions of South Korean retail investors used broker loans to buy the equities of tech giants as well as highly volatile…