NEW YORK — There’s nothing quite like a terrific dress. And Zac Posen knows just how to deliver one, which he proved in spades in the flirty, ultra-charming spring collection he showed on Thursday. The frocks ranged from the simplest slips to a soft jade fairy dress with sheer chevrons perfect for a young starlet. And Posen did great-looking day pieces, too — including some smashing blouson jackets and ruched skirts. Here, one of his slinky little numbers with a fluid skirt. For more on the season, see today’s reviews.