BOSTON (AP).- In an economic downturn, it might be tough to get your head around this: rare sheets of $100,000 bills, fabulous gold treasures dating back to the California Gold Rush era, rare coins including those tied to the first stirrings for America’s independence and federal government securities worth more than a billion dollars. That’s the backdrop of the country’s premier money show, the World’s Fair of Money, which has brought about 1,000 coin dealers and hundreds of collectors to Boston, seeking to tap into the surprising resilience of the coin industry. Held in a sprawling hall monitored by armed uniformed and undercover police officers, federal agents, private security contractors, electronic surveillance equipment and vigilant participants, the fair features seldom