PARIS — This city comes alive at night. Its pulse quickens as the museums empty out and the restaurants fill, the people crowding into bistros in the Latin Quarter, or the Marais or up on the hill near Sacre Couer. As evening falls, the 7th Arrondisement becomes the center as the Eiffel Tower begins to glow. It sparkles on the hour and — on most nights — shows the city’s heartbeat.
Friday was different. On this night, the light, the energy, the blood came from the southwestern part of the…