In a city known for high unemployment and Rust Belt decay, almost a hundred companies were quietly designing and building parts for components such as jet engines and repairing the planes that use them. The extent of the work being done impressed even people who knew some of the companies were there.  “The more that we began to research, the more that we began to go, ‘Holy smokes’ — the concentration here was amazing,” said Eric Voyles of the Rockford Area Economic Development Council.