Most days, Marc Hofer, a cybersecurity researcher and journalist based in Amsterdam, trawls the internet for clues about how China surveils its citizens, a subject that has fascinated him since he worked there as a foreign correspondent.Hofer, 46, was doing his usual scan earlier this year when he came across something called “Dynamic Control Platform for Overseas Personnel.” It was a futuristic dashboard, like something from the movie “Minority Report,” that appeared to track foreigners in the northern Chinese city of Zhangjiakou, a popular skiing spot that cohosted the 2022 Winter Olympics.The system’s dashboard said it tracked more than 700…