I was in the grips of a full-on Sunday hangover, on a caffeine-addled regret-stroll through East Williamsburg, when I locked eyes with a bar I’d never seen before, The Topaz. I wondered if the name, bestowed on this cocktail and small plates establishment, might be a play on “tapas.” Perhaps. But it also betrays the spirit of the place: a glimmering but none-too-fancy oasis in an otherwise spartan stretch of the neighborhood.