The summer blockbuster is dead — or at least, it has ceased to exist as a distinct entity. Four decades after "Jaws" set the template for mass-market Hollywood spectacle, the so-called event movie has expanded its turf so dramatically that July and August, once the most fiercely contested box-office battlegrounds, are now mere blips in the calendar. Each year, the blockbuster season seems to start a little earlier: the first major contender of 2016, "Batman v Superman: Dawn of Justice," opened way back in March.