Almost two years ago, two tightly packed pressure cookers unleashed a storm of ball bearings and nails that ended three lives and ripped off legs at the crowded finish line of the 117th Boston marathon. Locals this week will once again confront those indisputable facts and many that are far more complicated as a 21-year-old college student stands trial for crimes that felt to many like an attack on the city itself. The accused bomber, Dzhokhar Tsarnaev, must be concerned first and foremost with the opinions of 12 jurors and six alternates expected to be selected on Tuesday.