SPOILER ALERT: This story contains details about tonight's Season 1 finale of Legion. With a nod to Marvel’s big-screen tendency to feature a pivotal post-credits scene, tonight’s Season 1 finale of Legion found the powerful mutant portrayed by Dan Stevens captured and trapped perhaps even more than he was at the beginning of Noah Hawley-created FX series — and he was in a mental hospital and under government surveillance back then. “Going forward I think that the…