A construction worker frantically woke residents of a two-story home in Cambridge, alerting the three people upstairs to escape as flames swept through the second floor, grateful renters said this morning. Dozens of firefighters responded to the multi-alarm blaze. Ladder trucks poured water into the second floor, shattering windows. Water cascaded down the stairs inside the house and flooded the narrow residential street. One firefighter became briefly trapped upstairs, just inside the rear porch, triggering a mayday call and an automatic additional alarm, the fourth alarm sounded for the blaze, Mahoney said.