Not long ago, 20,000 textile and garment factories were bustling here, crowded with workers knitting and sewing for six, and sometimes seven, days a week to produce the wares sold at big American retailers like Gap and Wal-Mart. Now, demand is waning in the United States, and Shaoxing, a coastal city that is one of the world’s biggest textile centers, has fallen victim to the global downturn.