The new $2 million colonial near Wellesley Center comes with plenty of the features considered standard in this high-end community. A grand foyer opens into an oversized living room with an elegant bluestone fireplace and surround-sound speakers. It boasts six bathrooms, a granite-marble kitchen, first and second floor decks, and mahogany floors throughout. But there’s also something dramatically different -- it was prefabricated. Long known for cheap, drab construction, and tarnished by a trailer park image, some manufactured housing has gone upscale.