PYONGYANG, North Korea — North Korea marked its “Military First” holiday on Thursday with mass dancing, outdoor concerts and boasts of a successful — and potentially game-changing — submarine-launched ballistic missile test it hopes will serve as a warning to Washington and Seoul to stop holding joint military exercises Pyongyang sees as a dress rehearsal for invasion. Television news broadcasts and the front pages of morning newspapers Thursday showed images of the launch, conducted in the early hours the day before.