Angelique Jackson Sarah Ferguson, the Duchess of York, knows a little something about royalty and romance — and she’s bringing that expertise to the page with her debut novel, “Her Heart for a Compass.”Ferguson announced the news, posting a video message to fans early Wednesday, dressed in period costume and teasing the book’s premise, which is based on the life of her great-great-aunt Lady Margaret Montagu Douglas Scott.Already a prolific author of children’s books, biographies about Queen