Oil plunged more than 25%, 10-year Treasury yields dipped below 0.4%, stocks dropped, and currencies swung as the prospect of an energy glut ratcheted up turmoil across markets world-wide. Investors are responding to Saudi Arabia’s decision over the weekend to cut most of its oil prices and boost output, despite existing threats to demand from the coronavirus epidemic. The move escalates a clash with another major oil producer, Russia. …