JOHANNESBURG — Perhaps nowhere in today’s South Africa is the country’s inequality on more dramatic display than in the neighboring Johannesburg suburbs of Sandton and Alexandra. With its gleaming high-rises and lush estates, Sandton is known as Africa’s richest square mile. Alexandra, a onetime home to Nelson Mandela, is a squalid, cramped and crime-infested black township. Many of its residents stream into Sandton every day on a bridge over a highway to work in upscale shops or homes. Angry protests flared in Alexandra last month, stoked in part by campaigning for Wednesday’s national election but mostly by the frustration that South Africa should look far different than the country of haves and have-nots that it has become. Many voters believe the ruling African National Congress has lost its way since Mandela won the first post-apartheid presidential election in 1994, and that belief threatens the ANC’s absolute majority grip on power. The ANC has been shaken by widespread allegations of corruption that saw former President Jacob Zuma forced out a year ago, and many South Africans feel the party can no longer coast on its legacy of fighting the brutal system of apartheid. Unemployment in the country of 56 million people…