It took Seahorse, the new seafood spot in the W Hotel, four years to open after it was first planned — and only two weeks to conquer Manhattan. The wonderful eatery from owner John McDonald and chef John Villa is a rare, instant artistic and commercial success that’s just what Union Square needed. The blocks around the park lost their culinary cred after Brazilian-style Coffee Shop closed in 2018 and Bluewater Grill a year later, leaving the famous Greenmarket its only claim to glory. Seahorse,