How to keep a marriage vital? That’s the theme of “Yours Unfaithfully,” which was never staged since British playwright Miles Malleson penned the story 80 years ago. The subject is as difficult to write about compellingly as it is to produce. But the Mint Theater’s world premier does a splendid job dealing with this age-old question, suggestively recalling the candor of Thomas Hardy’s “Jude The Obscure,” which brought impassioned scorn to the novelist just two generations before Malleson created his vision of union.