CAMBRIDGE — Harvard University President Claudine Gay, the daughter of Haitian immigrants who rose through the sharp-elbowed politics of higher education to become the first Black leader of the nation’s most prestigious university, is resigning, a source familiar with the situation said. She is stepping down after months of contending with interlocking crises over the Israel-Hamas war, campus antisemitism, and allegations of plagiarism in her scholarly works. Harvard’s provost, Dr. Alan Garber, will become interim president, the source said.