In Havana’s late afternoon, when the heat had risen to more than ninety degrees, Maykel Molina Gutiérrez got off the bus in the posh neighborhood of Vedado, walked to an apartment building, and climbed six flights of stairs. Thirty-four with a hint of a beard, he kept a black backpack slung over his shoulder. By the time he reached the top, his ­T-shirt was drenched in sweat, and he used a corner to dab his forehead. He rang the doorbell—once, then twice.

No one answered. He pulled a beaten-up notepad out of his pocket and sighed. It would be…