LONDON — British prosecutors charged a former senior police officer with manslaughter Wednesday as they announced the first criminal cases in the 1989 Hillsborough stadium disaster, which left 96 people dead — many of them crushed against metal fences — and changed English soccer forever. The deaths initially were ruled accidental — a ruling overturned in 2012 after a new, wide-ranging inquiry. Files were sent to prosecutors to consider criminal charges and they announced their highly anticipated decision Wednesday. The tragedy at the stadium in Sheffield unfolded when more than 2,000 Liverpool soccer fans flooded into a standing-room section behind a goal, with the 54,000-capacity stadium already nearly full for the match against Nottingham Forest.