The average American teen now spends 8 hours and 39 minutes a day on screens, up two full hours since 2015. The Surgeon General issued an advisory this year warning that kids spend as much time staring at them as they do sleeping. Run that usage forward and by his 15th birthday, the average kid has already spent his 10,000 hours, the number musicians treated for a century as the price of mastery, handed to a slab of glass instead. The guitar never stood a chance in that fight. Learning it…