NEW YORK — Stocks plunged Wednesday as the United States edged closer to an unprecedented default on its debt. The Dow Jones industrial average was headed for its worst weekly decline in nearly a year. The Dow fell 207 points, or 1.7 percent, to 12,292 in afternoon trading, its biggest one-day drop since early June. The S&P 500 fell 25 points, or 1.9 percent, to 1,307.