The end of the dot-com boom may mean a decline in the market for spec mansions and starter castles. But there will always be millionaires, and the dog will always be man’s best friend. Until that changes, America’s luxury doghouse builders — who fill what must be one of the construction industry’s more specialized niches — will always have work. In fact, luxury doghouses are just a small segment of the growing pet-care industry. According to The New York Times, among the businesses that have recently spring up in New York City are Doggy Village, a $1,000-a-year health spa for dogs, and Pet Chauffeur, which maintains five minivans to deliver dogs to their acupuncture and massage appointments. Ron Smith, a