Music-industry survivors are supposed to look like Johnny Cash and sing blues-inflected tales of hard-earned wisdom. Paul McCartney has suffered along his own long and winding road - from the murder of his best friend to the death of a beloved spouse to a tawdry, tabloid-scented divorce from his second wife. But the McCartney who is at Fenway Park this week for two shows looks almost as buoyant as the mop-top who stepped off the plane from Liverpool 45 years ago. McCartney may have lost some gravitas following the Beatles’ break-up and stopped appealing to younger fans after Wings dissolved in the ’80s. But he kept on singing his trademark mix of boy-group optimism and soulful balladry. And he works hard to maintain a facsimile of his teen-heartthrob appearance, along with his boyish-jokester personality. For the tens of thousands of aging-obsessed baby boomers who will watch his shows, McCartney will have the useful effect of making 45 years seem like a short time. Survival doesn’t require the acknowledgment of pain, just the strength and desire to keep on working.