The final “Harry Potter” may not have had the magic to stay No. 1 at the domestic box office past its first week. But worldwide, the film has now crossed the $1 billion mark, a feat only eight other films have accomplished. “Harry Potter and the Deathly Hallows: Part 2” reached the monetary pinnacle in 19 days, tying “Avatar’s” record for the speediest arrival to that point, according to Box Office Mojo.