MARTHA Stewart, in cashmere and khaki, went next door to the John Jay Homestead Fair in upstate Katonah on Sunday. She glided through the main barn, cruised the grounds and checked out the little pigs just before race time. Stewart, diamond earrings glittering, leaned into the truck to get a gander at Lindsay Lo-ham and Britney Spear-ribs. The doyenne of domesticity, who knows a thing or two about getting penned in, said “Poor things” with a little pout and sailed on. Lo-ham took first place.