Arata Sushi swiftly slices its niche in the top tier of Long Island's inventive, modern Japanese restaurants. Jimmy Lian wields the knife. Lian, a young veteran of the Nobu empire, gets your attention with sharp sashimi and sushi, eye-catching and updated. His show is under way at the former site of Tsubo. The changeover is remarkable, for the seafood and the style.