For months, speculation has been building online that a buzzy horror novel, “Shy Girl,” was written with the help of A.I. Earlier this year, Max Spero, the founder and chief executive of Pangram, an A.I. detection program, heard of the claims about “Shy Girl” and decided to run a test of the full text. Its results indicated that the book was 78 percent A.I. generated. “I’m very confident that this is largely A.I. generated, or very heavily A.I. assisted,” said Spero, who posted his research on X in January.