TUBE TALK

         You knew that he had to turn up eventually. Fox announced yesterday that David Duchovny's Agent Mulder would return from wherever he's been all season for the two-hour series finale of ''The X-Files'' on May 19. Duchovny will also direct (but not appear in) the April 28 episode, based on a story he hatched with creator Chris Carter and executive producer Frank Spotnitz.
        The network says the series' wrap-up episodes ''will examine many of the pieces involved in the long-running mythology that has continued throughout the show's nine seasons.'' That's ''examine,'' not ''resolve,'' since untied loose ends will leave room for another ''X-Files'' feature film. So the truth will still be out there....