They’re part of the Los Angeles streetscape, as familiar as the swaying palm trees and idling traffic, living under freeways, alongside riverbeds and on canyon hillsides. The mentally ill, the drug addicts, the economically disadvantaged, many with their life belongings in a backpack or shopping cart. Here, The Times launches Without a Home, a special endeavor to examine a crisis of homelessness in our region. It is a challenge for each and every one of us. Now, city leaders and others must act to improve the plight of some 58,000 of the county’s most vulnerable residents.