In the Sudanese capital, civilians cower behind interior walls, fearing flying window glass and stray bullets. Commerce in Khartoum has ground to a halt. Water and electricity outages are widespread. Pitched battles have broken out near the airport and other parts of the city of 5 million, with fighter jets roaring overhead. Two military factions -- once political allies, now battlefield adversaries -- are wrestling for control over the city and over the country’s lucrative mineral resource exports, especially gold. Violence is erupting in…