In the world of the new movie Fingernails, any two people who believe they have fallen in love can take a test to prove it. It’s simple enough: Each partner must rip out a fingernail, wait a few minutes for a microwavelike contraption to inspect it, and voilà! A result. A 100 percent score means a perfect match—and peace of mind. Fifty percent indicates that only one person is in love. And zero? Well, you know. It’s the kind of kooky premise that brings to mind the techno-fables of Black Mirror and Yorgos Lanthimos’s satirical, dystopian romance The Lobster. But Fingernails, now streaming on Apple TV+, is not easy to classify. The Greek director Christos Nikou’s first English-language feature is neither a sci-fi parable with a body-horror bent nor a deconstruction of the romantic comedy. Rather, it’s a cheeky, gentle, and gracefully performed meditation on that perennially maddening question: What the hell is love anyway? That’s what the protagonist, Anna (played by Jessie Buckley), is constantly thinking about. She’s one of the lucky few who …