“Even at age 36, Lang Lang projects a boyish charisma that employs your protective instincts — all the more so if you saw him grow up before your eyes, emerging from his cramped Spruce Street apartment, speaking broken English, and yet becoming something as close to a rock star as any classical pianist can be.” David Patrick Stearns (who did see all that) reports on Lang Lang’s performance of a Mozart concerto with the Boston Symphony last week and checks in with Lang Lang’s primary teacher at Curtis, Gary Graffman.