Leighton Meester vaulted into the pop cultural vanguard in fall 2007, joining the fictional likes of Kelli Taylor, Cordelia Chase and Summer Roberts in television’s pantheon of razor-witted, solipsistic high school Alpha females. Meester made herself “It Girl" du-jour as the anti-heroine Blair Waldorf in The CW’s teen soap “Gossip Girl” (2007- ), American TV’s latest showcase of its obsession with spoiled, angst-ridden rich kids. Already able to boast a relatively voluminous list of credits for a 20-something up-and-comer, Meester budded into TV’s newest überdeb, playing the private-school