Ayatollah Ali Khamenei, Iran’s slain supreme leader, had seemed immovable for decades, a man whose authority had become so deeply woven into Iran’s political and religious life that imagining the country without him felt almost impossible. Now Tehran — the capital from which he ruled, where he was killed and which had shaped his life — is the center of his final journey, filled with mourners for funeral ceremonies taking place across several days, which are part farewell, part spectacle and part turning point. In the days leading up to the first public mourning, the city changed. First gradually, and...