“THIS IS NOT A RETROSPECTIVE,” warned Ivor Davies as I walked into the octogenarian’s mega-exhibition, up through March 20, at the National Museum Cardiff. An unsuspecting visitor to the show—dedicated to one of the Welsh art scene’s leading lights and crammed with paintings, sculptures, and archival materials—might be forgiven for wondering, “Why not?” Certainly, “Silent Explosion,” curated by the museum’s Nicholas Thornton and by scholar Judit Bodor, has all the makings of a good, long, reflective survey. The exhibition, described by the museum as “the first to consider the broad