After posting a cryptic cartoon for fans last week, Alvvays today released the excellent strum-n-thrummer “In Undertow,” a breakup fantasy that is both crushing and charming for its level-headedness. “You find a wave and try to hold on for as long as you can, you made a mistake you’d like to erase and I understand,” sings Rankin, her voice full longing, not for another person necessarily, but for what to do next. “Meditate, play solitaire, take up self-defense,” Molly continues, laundry-listing some strategies for moving on. “What’s next for you and me? I’ll take suggestions,” she deadpans under crashing waves of Farfisa and feedback.