Sharp lines define this season's geometr y lesson. THIS PAGE: Dress. $5,145. Versace. 888-721-7219. Necklace. House of Lavande. Watch, Cartier. Cuff. Hervé Van Der Straeten. Ring, Vherniec. OPPOSITE PAGE: Swimsuit. $396, MichacI Kors. 866-709-KORS. Hat. Marc Jacobs. Cuff (left). Susan Hanover. Ring