Here’s the plan: You’re an apex superstar tasked with one-upping the most lucrative pop tour our planet has ever seen, so you travel to Sweden to build a sunshine bomb with god-level hitmaker Max Martin, BYO photons and fructose. Foolproof, right? Eh. Instead, Taylor Swift has returned from the feel-good factory with “The Life of a Showgirl,” an album about feeling blah, feeling bitter, feeling nostalgic, feeling vindictive, feeling loved, feeling performatively horny, but ultimately, still feeling things out. “Showgirl” isn’t some redux of “Red,” that tidal wave of serotonin that Swift invited Martin and his studio partner Shellback to surf with her back in 2012. But it is a satisfying pop album blessed with the hygienic, aerodynamic production qualities required to squeeze a superstar’s mixed emotions into legible frames.