“The thing about an urban legend is that it never happened to the person tellin’ it. It always happened to someone else.” So intones Kimberly Gilbert at the top of The K of D, Laura Schellhardt’s spooky, richly-layered mystery, now in its world-premiere run at Woolly Mammoth Theatre Company. The show is less a whodunit? than a whatthefuk?, though who-done-what is necessarily foremost on Gilbert’s mind: She plays all 12 of the play’s roles,…