Like the “fusions” that terrify the main characters, Dean Koontz’s new thriller, “The House at the End of the World,” feels not quite fully formed. It starts as a mystery — what sort of dangerous experiments is the U.S. government conducting on an island called Ringrock? — and ends as a buddy story, with a woman and a teen on the run, paranoid that they are being hunted for what they know about Ringrock.