Leftist mayoral candidate Zohran Mamdani has a . . . remarkable idea: Make sure even more mentally ill homeless people are constantly thronging the subways. Yes, Mamdani wants to convert empty retail spaces in the MTA system into city-run facilities offering drop-in services for the homeless and outreach toward longer-term solutions. His vision, as he smarmed to a fawning interviewer, would provide “a moment of relief, a moment of care, a moment of guidance” to homeless people.