On a street corner in the heart of Saint-Germain-des-Prés, Allard has been a quintessential Paris bistro ever since it opened, in 1932. A few weeks ago, though, these two tiny sepia-toned dining rooms lost something of their permanent Édouard Boubat aura with the installation of a shiny new overhead décor of beautiful soldered-and-fitted ductwork and high-tech air filters and fans that was created to ensure an air quality on par with that of a hospital operating room.Or so I’d been assured…