Today marks the 75th anniversary of the Mercury Theater on the Air’s radio production of “War of the Worlds” — a dramatization of a Martian invasion that put both Orson Welles and Grovers Mill, N.J. on the map. A recent PBS documentary discussed the broadcast and the fallout from it — angry listeners who thought the whole thing was real, that Martians were using poison gas and heat rays to rip up a good chunk of the East Coast, and that civilization as we know it was as good as roasted — and on the day before Halloween, no less.…