PHILADELPHIA — Philadelphia will soon have a new mayor, a new police commissioner, and dozens of new employees in key parts of city government. All will inherit a gun violence crisis that is at an unusual crossroads: Shootings are plummeting, but they’re also still at levels well above recent historical norms. The number of homicides in the city in 2023, for example, is down about 20% compared with last year, police statistics show — mirroring a decline in violence nationally and rivaling the largest single-year declines in Philadelphia over the last 60 years. Still, with more than 400 people …