Airbus Group NV threw a party for its new U.S. workers ahead of the formal opening Monday of its first commercial jet factory in the United States, and representatives of the machinists union joined the crowd. The machinists’ low-key presence highlighted the new Airbus factory’s significance to a commercial aircraft industry that’s in transition from a unionized, centralized past to a future where rivals Boeing Co and Airbus build planes around the world, in factories that may no longer be union shops.