MILAN — Talk about a trend with staying power. Satins, nylons and metallics have been all over the runways for several seasons, and the glow shows no signs of dimming. Even Jil Sander stepped into the light with the collection she showed Tuesday, sending out a group of iridescent metallic pantsuits that crossed elegance with cool. For more on Sander and Milan, see pages 6 to 8.