More than two decades after the Exxon Valdez supertanker struck a reef and unleashed the nation’s biggest tanker spill, a lingering legal dispute about the disaster heads back to court on Friday. At issue in a U.S. District Court hearing in Anchorage is an unpaid $92 million claim by the U.S. Justice Department and the state of Alaska for what they consider long-term environmental damage unexpected at the time of the grounding.