Shipping firms shut down 29 West Coast ports this week over a labor dispute with longshoremen, leaving dozens of ships bobbing offshore with no place to unload. The freeze will cost the nation $1 billion a day in lost wages and sales. If it lasts, economists said, it could spark shortages of everything from cars to machine parts, and keep stores from stocking shelves for the holiday season. The companies accused dockworkers of forcing their hand by slowing down operations after five months of contentious contract talks. Union spokesmen called the tactic heavy-handed, and unfair to 10,500 longshoremen ready to work. This is a major economic disaster, said Robin Lanier, president of a coalition of importers including Wal-Mart, Kmart, and Toyota.