AMC Theatres will open a 12-screen movie complex at Assembly Square in Somerville, becoming the first major tenant to commit to opening in a $1.5 billion development on the former industrial property. Developer Federal Realty Investment Trust said yesterday that the theater will open on the third floor of a large retial building that will also include several restaurants and shops. It will replace an AMC-owned theater that shutdown on adjacent property in 2007. The new operation will feature stadium-style seating and 100 percent digital production, and AMC may also incorporate an IMAX screen and bar service, according to Federal Realty. The developer said construction will start later this year, with the theater planned to open as early as the fall of 2013.