VAIL — Stan Zemler stops his stroll through Vail Village to gather random detritus from the heated brick streets. “It’s one of the things I try to always do,” he says, cradling cups and cans plucked from planters, sidewalks, gutters. It’s safe to guess that Zemler won’t stop combing Vail’s streets when he steps down from his town manager gig next month after more than 13 years of piloting the resort community through a $2 billion renaissance and a belt-tightening recession. Like the skiers on the hill that towers over town, Zemler’s up-and-down ride — the longest tenure for a town manager in Vail history — has had its thrills and challenges.