At the Pit, a Night Shift to Numb the Body and Soul
Ironwrokers, firefighters face extra physical and psychological challenges working overnight shift to clear debris from underground levels of collapsed World Trade Center towers; have removed estimated half of rubble, or about 600,000 tons of steel and debris, but count their success not in metal or tonnage, but in number of bodies recovered; for them, work remains recovery operation and will be to end, despite pressure from contractors in charge and decrees from City Hall; photos (M)