HAMBURG — Cristiano Ronaldo has used it. Lady Gaga, too. You can count in Ariana Grande, Kendall Jenner, Harry Styles and Hailey Bieber as well. And let’s not forget all those less famous people who’ve called themselves a “Pilates Princess” on TikTok: They all work out on the reformer, possibly the most sought-after piece of fitness equipment at the moment. For the latest news & views from every corner of the world, Worldcrunch Today is the only truly international newsletter. Sign up here. This 100-year-old machine originally comes from Germany — and in its classic form could also be found in…