Homelessness is a soaring public health crisis, with a record 653,000 unhoused people in the United States, according to federal estimates. Tent and recreational vehicle encampments have exploded in recent years, crowding streets and sidewalks from Portland, Ore., to New York. In California, where roughly a third of all the nation’s homeless people live, doctors are scheduling appointments at encampments to treat widespread chronic disease, rampant drug abuse and mental illness — health conditions that worsen with homelessness.