The Paul McCartney the World Has Never Met: In Mary McCartney’s New Documentary, the “Beatle” Disappears and What’s Left Is a Dad Humming Half-Finished Melodies in the Kitchen, a Grief-Stricken Survivor Who Outlived an Era, and a Tender, Funny, Almost Fragile Man Whose Quietest Moments Finally Reveal the Heart Behind No More Lonely Nights — and Early Viewers Swear It Feels Less Like Watching a Film and More Like Being Invited Into the Living Room He’s Spent a Lifetime Protecting