Madonna stood in a silvery bodysuit on an arena stage, having just landed back on Earth after flying around in a suspended box while singing “Ray of Light.” Her audience’s eyes were still recovering from her high-wire gyrations, her postapocalyptic backup ravers, and her many, many lasers. But there was a new reason to gasp: A black-robed figure was creeping up behind Madonna. Death was coming for the queen of pop. Thoughts of mortality were already in the air. She was originally supposed to kick off the Celebration Tour—a greatest-hits revue—this summer, but a bacterial infection put her in the intensive-care unit in June. She recovered, but, as she told the audience at Brooklyn’s Barclays Center on Wednesday—the postponed opening night for the tour’s U.S. leg—she really had feared she wouldn’t survive. In our youth-worshipping culture, Madonna’s mere existence as a 65-year-old woman who’s still in the spotlight already feels like a provocation, a performance. One might expect her to use this tour to assert her unkillability; instead she’s making impermanence part of the act. …