Williamsburg has officially traded its gritty hipster past for a luxury retail future — and investors are piling in. Prime corridors like North 6th Street are fetching record prices. Retail buildings have traded for up to $6,000 a square foot, while asking rents are pushing $400 to $500 a foot, on par with storied Manhattan retail corridors like Soho & Madison Ave. Institutional players like Acadia Realty Trust, Empire State Realty Trust and City Urban Realty have scooped up millions in North Br