Cuba is running on fumes. Across Havana and its outskirts, the signs of strain are everywhere: quieter streets, fewer cars, garbage piling up and rolling power outages. Last year, Cuba’s fuel imports fell by more than a third, and electricity shortfalls have at times wiped out nearly 30 percent of daily demand. Years of economic deterioration have tipped into something more acute. A tightening U.S. trade embargo, the collapse of Venezuelan fuel deliveries and pressure on countries that attempt