The ostensibly fictitious hard-rock band Spinal Tap has been together, off and on, slightly longer than Katy Perry has been alive, and as of Friday, has released four studio albums and two feature films. Their latest album, “The End Continues,” continues to do what they’ve always done best: delightfully walk the fine line between clever and stupid. Its release coincides with the sequel film, “Spinal Tap ll: The End Continues.”When the mockumentary “This is Spinal Tap” and the accompanying debut album were released in 1984, the idea of a band rocking into middle age still felt mildly ridiculous. As Spinal Tap, their earnest belief in the everything-ness of rock ‘n’ roll is at times riotously funny but ultimately endearing. They muse on Satan’s immortality in a few lyrical lines: “He’s not starting to lose his teeth / He’s not starting to eat less beef,” goes one.