Somewhere between a helicopter and a dream, Joby Aviation is asking investors to believe that the skies above American cities will one day carry electric air taxis — and that Joby will be the company carrying them. The company leads a field that does not yet commercially exist, holding more test flight miles and a more advanced regulatory standing than any competitor, yet still separated from profitability by years of manufacturing scale, regulatory approval, and market adoption. A tenfold return on its stock is mathematically possible, but it requires a long and unbroken chain of conditions —