SEAL Cmdr. Chris Cassidy was one of the first Americans on the ground in Afghanistan after the Sept. 11 terrorist attacks. He led boarding teams in the Persian Gulf. And now, he’s in the former Soviet Union. But that’s just the beginning of his next mission — one that makes trips across the globe seem like a walk to the corner store. Cassidy will serve as a flight engineer for a three-man crew aboard a Soyuz spacecraft set to launch March 29 from Kazakhstan to the International Space Station, where the crew will stay until mid-September. It’ll be the second…