“One of these days, we’ll never be apart again,” John Paul Pitts croons at the climax of “Six Flags in F or G,” a nerd-rock barn burner built on spaghetti western guitar riffs and wah-wah soloing. Within this manic context—of both the song and the band itself—it’s easy to miss the lyric’s subtle poignancy. Snowdonia, Surfer Blood’s inventive fourth LP, is their first without…