As Diversify rounded the final sweeping turn at Belmont Park in Saturday’s $750,000 Jockey Club Gold Cup (G1), Rodney Paine, perched on a platform above the finish line, started yelling. “Come on, Big D!” he exhorted. “Dig in! Dig in, Big D!” And dig in did the four-year-old son of Bellamy Road. Having led largely…