Across Paris’ toniest shopping districts, luxury boutiques bear the scars of a weekend of rioting that filled social media feeds around the world. Now they are bracing for the possibility of another round. On the avenue Champs-Elysees, Louis Vuitton’s flagship store has metallic grates protecting its windows that are not there at quieter times, as does the Christian Dior Couture emporium on avenue Montaigne, a nexus of high-end shopping in Paris’ eighth arrondissement. Both stores escaped...