SEATTLE — Boeing factory workers voted Wednesday to reject the company’s latest contract offer and to continue a six-week strikethat has halted production of the aerospace giant’s bestselling jetliners. Local union leaders in Seattle said 64% of members of the International Association of Machinists and Aerospace Workers who cast ballots voted against accepting the proposal. “After 10 years of sacrifices, we still have ground to make up, and we’re hopeful to do so by resuming negotiations promptly,” Jon Holden, the head of the IAM District 751 union, said in a statement Wednesday evening.