KARACHI, Pakistan — Five years after most senior al-Qaeda leaders are thought to have fled this port city, officials in Karachi worry that the organization is regrouping and finding new support here and in neighboring Afghanistan. They are especially concerned about the recruitment of potential foot soldiers for the next major terrorist attack. The resurgence has been managed by a South Asian offshoot called al-Qaeda in the Indian Subcontinent (AQIS), created by al-Qaeda’s top leader, Ayman al-Zawahiri, in 2014 in order to slow advances by rival Islamic State militants in the region….