“Set aside their cult following, and the title of indie-punk duo Shellshag's third LP, Shellshag Forever, is still well earned. John Shell, who is 45, and stand-up drummer Jen Shag, 42, have been recording pop tunes with K Records-style teen-spirited emotion for over 15 years (the two are a couple), having earned their stripes trekking the DIY circut since the cellphone- and GPS-less days of the 19”