If there were an aptitude test to qualify for a job at X, Alphabet Inc.’s renowned research lab, Kathryn Zealand would ace it. She’s a McKinsey & Co.-trained entrepreneur who abandoned a Ph.D. in theoretical physics to build startups in sub-Saharan Africa. In her spare time, she climbs California’s tallest mountains while bingeing sci-fi audiobooks. She joined X in 2018, and for the past five years, she’s dedicated her life to building an exoskeleton.On a recent afternoon in San Francisco, Zeala...