China has a knack for putting stars in commodity executives’ eyes, and then knocking them out. With oil once again under pressure—global benchmark Brent is down 10% since the beginning of March—investors are firmly focused on buoyant U.S. supply as the main risk factor for further declines. But as with a few years ago, it is weaker demand from China that could deliver a punch to staggering oil markets. Late 2013 and most of…