A roomful of fidgety second graders fell silent when wilderness author Tim Caverly started the ghostly tale of a 150-year-old lumberjacks’ boarding house along the Allagash Wilderness Waterway. When he writes, it’s like having a movie playing in his mind, Caverly told the group at Houlton Elementary School. “I try to write so you can read a passage and close your eyes and picture what you are reading,” he said during a recent student session a…