Waiter, Hold the Foie Gras: Slump Hits New York Dining
New York City restaurants, which for months seemed impervious to economic downturn, have suddenly seen phenomenal double-digit growth in sales come to screeching halt; unluckiest have seen business drop as much as 30 percent, and top restaurants whose stature makes them least vulnerable to cooling economy are starting to feel chill; on brink of fall season, traditionally time when lavish new places open and luxury factor is pumped up on menus across city, restaraueurs are instead worrying about filling tables in oversaturated market; photo (M)