As the economy continues to plummet, holiday giving is down, say the experts. But that's not the case at the Somerville Housing Authority, where last week the owners of Cataldo Ambulance helped organize their annual toy drive for the youngest residents of the city's public housing complexes. On Tuesday at 530 Mystic Ave., housing authority employees will organize a wrapping party to wrap all the donated packages kids will happily tear apart on Christmas morning.