“What the courts may settle is this: Is copyright ownership enduring and inviolate, as old media companies believe? Or is it a more transitory function that technology is overrunning anyway, even as its weakening will ultimately be good for everyone? And doesn’t our mash-up era, in which compelling new works of art are created by combining perhaps less compelling old ones, prove the point that you can’t really own an intellectual idea, even if copyright law thinks you can?”