Frieze lasts only four days, but the museum shows timed to it run for months, so a short trip is no reason to feel shortchanged. Seoul's institutions save their biggest openings for the first week of September, when collectors and curators are in town. Here are five worth planning a day around after a long afternoon of booth-hopping. The biggest of the lot opened Thursday at the National Museum of Modern and Contemporary Art, Korea in Samcheong-dong: the largest show yet by arguably the most in-