Three full-floor residences in the neo-Georgian-style Whitman building on Madison Square Park have sold, but the historic structure’s penthouse is still available. Priced at $25 million unfurnished, the home offers 3,106 square feet of private outdoor space and a recently finished interior design scheme by the emerging designer Jeremiah Brent. Brent’s interiors, with furnishings that he describes as “the new traditionalist,” complement the unit’s light oak floors and white marble counters. Vintage pieces from …