Once a butterstick, now a “solid little Tonka truck,” the baby panda that has transfixed attention on the District’s National Zoo and provided much-needed fodder for slow news days is six-weeks old today, almost half-way to the 100 days at which it is officially named. Zoo vets examined the cub yesterday for 14 minutes, reports the Post, weighing it in at four pounds and noting that teeth and open eyes are the next developments…