NEW YORK — It’s all in the jeans — designer jeans, that is. Denim is turning up in a variety of playful incarnations for spring, including pieces that are cropped, patched, embroidered and beaded. Here, from the inventive collection he showed on Monday, Marc Jacobs’s luxurious basics: abbreviated cotton jeans, worn with a cashmere T-shirt. For more on the season, see pages 6 to 9.