It has been seven years since Hurricane Katrina brought New Orleans to its knees and a month since Isaac threatened to push the city down again. But the region’s new $14 billion levee system held back the tide and within hours of Isaac’s departure the city sprang back to life to host Labor Day visitors. A deeper sign of the city’s resilience can be found in the national developers and retailers now coming to town as first-time investors. Among them is the Howard Hughes Corporation, which is based in Dallas and plans a $70 million renovation and expansion of Riverwalk, a struggling riverfront shopping and entertainment complex. In July, the Fresh Market, an upscale food chain based in North Carolina, opened a 24,800-square-foot grocery store in a historic building on St. Charles Avenue. And at the opposite end of the retail spectrum, Costco, the national warehouse chain, has committed to open its first store in Louisiana in another part of New Orleans.