Three years after opening its always-packed Soho store, J.Crew’s younger, jeans-obsessed sister Madewell unveils its largest outpost yet on Tuesday in the Flatiron (115 Fifth Ave., at 19th St.; 212-228-5172). For the next two weeks, the store will house a “Hometown Heroes” shop-within-a-shop, selling merchandise from more than a dozen local indie designers like Erie Basin, Top Hat, Bowery Lane Bicycles, and Other Music, in addition to the house line of colorful jorts, silky tanks, jute wedges, and bikinis.