Zohran Mamdani, a little-known state lawmaker whose progressive platform and campaign trail charisma electrified younger voters, stunned former Gov. Andrew M. Cuomo in the Democratic primary for mayor of New York City on Tuesday night, building a lead so commanding that Mr. Cuomo conceded. Mr. Mamdani, a 33-year-old democratic socialist from Queens, tapped into a current of anxiety around New York City’s growing affordability crisis. His joyful campaign brought new voters into the fold who...