HONG KONG — The famine that gripped China from 1958 to 1962 is widely judged to be the deadliest in recorded history, killing 20 to 30 million people or more, and is one of the defining calamities of Mao Zedong’s rule. Ever since, the party has shrouded that disaster in censorship and euphemisms, seeking to maintain an aura of reverence around the founding leader of the Communist state.