Enrico Casarosa's “Luca” — a summery, shimmering fish-out-of-water fairy tale — is one of Pixar's most pure and condensed enchantments.Pixar has plunged into the sea before, of course, in the aquatic “Finding Nemo” and “Finding Dory.” Lushly detailed waters have been sprinkled through many of the studio's films, from the rushing river of “The Good Dinosaur” to the frothy seaside surf of “Piper.” One personal favorite: how, after the frantic Paris chase in “Ratatouille,” the diminutive Chef