By Alex Lauer When you go to light off your stockpile of fireworks this Fourth of July weekend, take an extra moment to consider where those mortars, firecrackers and Roman candles came from. No, we’re not talking about the pimply teen who sold you the 36-shot Gallantly Streaming aerial repeater with the waving American flag on the box. We’re talking about the little-known Chinese factories where the overwhelming majority of consumer fireworks are made. “You don’t know until you know,” says Jessi Dragoiu, talking about China’s historic pyrotechnic industry, which many Americans are blissfully …