Baltimore — In the shadow of a summer block-party massacre that left two dead and nearly two-dozen wounded, a quiet but determined effort has taken root in one of the city’s most embattled neighborhoods. The program, known as Safe Streets Baltimore, seeks to treat gun violence not merely as a crime problem but as a public-health emergency — one that can be interrupted, redirected and healed. Launched in 2007, Safe Streets was modeled on the Chicago-based Cure Violence interve