Nearly 100 years ago, outside the old city limits of Wilmington, N.C., Mary Bridgers began a novel experiment in residential development now known as Carolina Heights.    The neighborhood she began has grown and matured, but remains a jewel-box settlement of only 12-blocks, rich in a variety of turn-of-the-century architecture. Each street and throroughfare offers glimpses of magnificent mansions as well as smaller dwellings whose designs were directly influenced by Frank Lloyd Wright.   Home to a diverse culture of Wilmington residents, Carolina Heights has resisted the trends to change the appearance of her old homes, churches and buildings. As such, the neighborhood serves as a hidden treasure of preservation, architecture, and historic homes and gardens.