It started late this morning with the arrival of a few past-their-prime camper vans and rusty station wagons with Vermont and Maine license plates. By lunchtime, one couldn’t help but notice all the ponytails, facial hair, and worn-out t-shirts. This afternoon, the people in front of me at the CVS checkout line were buying a healthy quantity of Cheetos and Visine. There can be only one explanation: Phish is playing at Fenway Park.