The sudden death of Iran’s president, Ebrahim Raisi, opens a new chapter of instability just as the increasingly unpopular Islamic Republic is engaged in selecting its next supreme leader. Mr. Raisi, 63, had been considered a prime candidate, especially favored by the powerful Revolutionary Guards. Even before the helicopter crash that killed Mr. Raisi, the regime had been consumed with internal political struggles as the supreme leader, Ayatollah Ali Khamenei, 85, the longest-serving head...