Harvard Square is a place where even the threatened passing of a Dunkin’ Donuts can prompt student outrage; passion for favorite food haunts runs deep. While both students and restaurants in the Square turn over as fast as patties at Tasty Burger, some local favorites persist. Enter the Grafton Group, one of Harvard Square’s largest locally-owned families of restaurants, which has been around since 1996. In the nearly twenty years since the group’s founding, it has been a witness to a Square in flux.