From the beginning, “The Designated Mourner” by Wallace Shawn should have been a podcast — except when it was written, in 1996, podcasts hadn’t been invented.The intimacy of the ideas in this urgently relevant parable about the perils of inaction and deliberate blindness requires intense listening. Shawn challenges audiences to stay afloat on the river of his words. Now Gideon Media has produced this prescient, disquieting, and rambling work in its rightful format.Although a three-hander, the play is a monologue …