VERNON, France — In a tightly controlled manufacturing hangar west of Paris, workers put the finishing touches on an enormous silver-colored engine. In just a few days, a similar machine will help propel the most powerful version of Europe’s Ariane 6 rocket yet, flying for the first time with four boosters. On Thursday, the Ariane 64 rocket — named after its four boosters — is scheduled to make its maiden launch from the European spaceport in Kourou, French Guiana, aiming to deploy 32