Stocks turned narrowly mixed around halftime Thursday, as the market showed little change and took a breather after two straight sessions of strong gains. The Nasdaq rose 0.2%, while the S&P 500 edged up 0.1%. Each had been slightly negative earlier. The Dow Jones industrial average lost less than 0.1%. Volume was tracking down sharply from Wednesday's levels across the board. Biogen Idec (BIIB) slid 4% in fast trade, but had found support at its