A powerful chain-reaction explosion ripped through Mexico’s best-known fireworks market on the northern outskirts of the capital Tuesday, killing at least 29 people, injuring scores more and sending a huge plume of charcoal-gray smoke billowing into the sky. Video of the blast showed a dramatic staccato of rockets exploding in flashes of light, leveling the open-air San Pablito Market in Tultepec in Mexico State as it bustled with shoppers stocking up on fireworks to celebrate Christmas and New Year’s.