Chicago Folks Operetta has never met a forgotten operetta it doesn’t like. Or so it would seem. Last summer the enterprising troupe treated “Arizona Lady,” a singing-cowboy rarity by Hungarian composer Emmerich Kálmán, to its belated American premiere. Now the good Folks are back, ensconced for the rest of the month at the charmingly intimate, if minimally air-conditioned, Chopin Theatre in Wicker Park, for what is billed as the first U.S. performances in more than 85 years of another seldom-heard confection, Leo Fall’s “The Rose of Stambul.” They do an engaging job of it…