The nation’s largest fireworks display exploded in a spectrum of color over the East River, temporarily stealing the spotlight from New York’s world-famous skyline and helping to create a brilliant end to a day of July Fourth celebrations nationwide. No crowd estimate was immediately available from police or organizers, but more than three million people had been expected to attend the display. It was moved south along the river this year so onlookers could get a better view of the skyline. Spectators thronged the riverfront in a light rain, some holding red, white and blue umbrellas. More than 35,000 shells sparkled, arched, spiked and fanned over the river during the half-hour show…