"Stand by Me." Julian Lennon. MCA. $29.95. This musical portrait takes tiny steps toward what should be the future of such videocassette projects: true documentaries that explain the artist's background, motivation and art. Mostly, however, "Stand by Me" is merely an extension of the souvenir programs sold at concerts--though it goes the printed booklets one better by including music from the concert. Besides seeing the extremely likable son of John on stage, we see him rehearsing, sleeping on the tour bus, joking with band members and everything else that is standard in this genre.