PORT-AU-PRINCE, Haiti — Paul Collier, a leading poverty guru, spent a recent morning here waxing positive about how the world's economic freefall might prove the perfect moment for Haiti to sell more exports like T-shirts and mangoes to Americans.  His improbable enthusiasm coincided with appearances by a bevy of luminaries descending on Haiti this month, including Ban Ki-moon, the United Nations secretary general, and the entire Security Council. All of them came to stress that this destitute nation stands at a crossroads between salvation and "the darkness," as Mr. Ban put it.