Jewelry retailer and direct marketer Tiffany & Co. Inc. yesterday said it acquired a 5 percent stake in Della.com and agreed to make the San Francisco online gift Web site its exclusive wedding-registry partner. The amount of the investment was not disclosed. Tiffany, New York, said the investment marks a significant step in its effort to expand into the wedding-gift market and establish access to customers at a critical stage in their lives. The agreement eventually will allow engaged couples to create Tiffany registries online at Della Weddings, Della.com’s wedding Web site. Della.com’s other partners include Amazon.com, Crate & Barrel, Dillard’s, Neiman Marcus, REI and Williams-Sonoma.