Sunday night’s “au revoir” party for “Les Miserables,” which gave up the ghost after 16 years on Broadway, began at the Imperial Theater, when more than 300 acting alumni of the Cameron Mackintosh megamusical took to the stage and marched in the aisles for a grand finale that rocked the house.Frances Ruffelle, Judy Kuhn, Randy Graff, David Bryant, Leo Burmeister and other stars of the original company joined current cast members on the barricades for the last time, as the French national colors rained down in ribbons of confetti and bursts of balloons. “Well, I guess that’s the end of our cultural detente with France,” general manager Alan Wasser noted ruefully. Social detente was closer to the mood at the after-party in the Summer Garden of Rockefeller Center, as clusters of Valjeans and Javerts nibbled on paper cups of French fries and swapped war stories with former Fantines and Cosettes. Kevin Wright, who played Marius in 1991, was mistaken for the Marius of the moment, Kevin Kern, who was in the company of another Marius look-alike, his brother Joey Kern. “It boggles the mind,” sighed a bystander, just as Bryant, the original Marius, walked by.