On Thursday, Britons will vote in a referendum on whether their country should stay in the European Union or leave it. If a majority opts for “Brexit,” a long earthquake begins. It will topple the old facade of Britishness. It will disrupt, perhaps mortally, the foundations of European unity. The sense of a fateful moment suddenly peaked on Thursday, when, the police say, a young Labour member of Parliament named Jo Cox was shot to death in her West Yorkshire district by a man who is said to have shouted, “Put Britain first!” and to have been involved in the white-supremacist National Alliance in the United States.