HONG KONG — In the dead of night last month, two tanker ships pulled alongside each other in the East China Sea. One was a North Korean vessel, the other was the Belize-flagged Wan Heng 11. Lights on both ships were blazing, arousing a Japanese spy plane’s suspicion they were carrying out a “ship-to-ship” transfer banned under United Nations sanctions imposed over North Korea’s nuclear weapons program. Records for the Wan Heng and a number of other ships identified in recent U.N.