Constance Bacon was walking home from the gym in a driving rain Monday, when she spotted a woman lying on the sidewalk on Beacon Street in the Back Bay, raindrops soaking her black pants and long wool overcoat. When Bacon called an ambulance, she said she had no idea it was Joan Bennett Kennedy, a member of one of America's best-known political families. Kennedy, 68, had apparently tumbled, injuring her shoulder and leaving a gash on her head. When Bacon found her about 5 p.m., Kennedy could not pull herself up from the sidewalk. So Bacon, 35, an artist who...