Boy-Bound Butterfly: For reasons known best to the butterfly, this one today picked out nine-year-old Robert Kirk of 2115 Alice St. for some extraordinary attention. He's pictured here on Bob's neck as Donald Poppenger, 5, Bob's half-brother, watches. Exactly what the butterfly's unusual attraction for Bob is, nobody knows. But the youth says the butterfly landed on him at about 9 o'clock this morning and just stays there, flitting from one spot to another. Bob says he'll keep the black beauty with yellow, orange and blue spots, as a pet. What else can he do?