Jon Stewart and "Star Trek," two of the most successful franchises in media history, are returning to television. But neither is actually going to be on TV. Stewart signed a four-year contract with HBO, and "Star Trek" has a new deal with CBS. Each development was big news last week for millions of fans. But taken together, they have something even larger to say to all of us: The digital future for TV is arriving a lot faster than many analysts thought. In fact, when it comes to the very best TV