A scrawny hippie and a nerdy engineer who became prank-playing friends vowed to change the world when they founded a Silicon Valley startup on April Fools’ Day 50 years ago and then — no joke — pulled it off. The improbable odyssey began April 1, 1976, when a then shaggy Steve Jobs and his gadget-tinkering friend Steve Wozniak signed a two-page partnership document that created Apple Computer Co.