Many poor families spend more than half of their income on rent. That means even a short period of unemployment can lead to homelessness in a hurry. As part of the KERA series One Crisis Away, this hour, we’ll talk about what it’s like to live life so close to the margin with Matthew Desmond, co-director of Harvard’s Justice and Poverty project and author of “Evicted: Poverty and Profit in the American City” (Crown).