Building trades spent the past five years preparing for an offshore wind boom that went bust. Now, U.S. workers mourn the well-paying careers they don’t see coming back.By Ajani StellaWhen Gerard Mullin decided to train in offshore wind, he didn’t know what to expect. But on his first long-term project in Massachusetts, he immediately relished the consistency of the work, the good pay and the novelty of laboring at sea.