YouTube’s Coachella livestream has been contemporary music’s greatest showcase: a proving ground for superstars, rising and returning artists — all of whom know they need to bring their A-game — that easily can eat up your entire weekend. “Couchella” stans know that you don’t have to spend thousands of dollars, inhale several pounds of desert dust, run a quarter mile between stages, or even leave your home to witness these world-beating performances.