NEW YORK — A preschooler toying with the burners on his mother's stove accidentally sparked New York City's deadliest fire in decades, an inferno that quickly overtook an apartment building and blocked the main escape route, the fire commissioner said Friday.A dozen people died, and four others were fighting for their lives a day after the flames broke out in the century-old building near the Bronx Zoo.The 3½-year-old-boy, his mother and another child were able to flee their first-floor apartment. But they left the door open behind them, and it acted like a chimney that drew smoke and flames into a stairwell. From there, the fire spread throughout the five-story building, authorities said.At least 20 people scrambled out via fire escapes on a bitterly cold night, but others could not."People had very little time to react," Fire Commissioner Daniel Nigro said.