Notes"SERIOUS cold, Justen Ladda said, is when the sponge in the kitchen sink feels like wood or the toothpaste freezes or the refrigerator turns itself off, as it did one particularly frigid day last winter. Not that Mr. Ladda, a 56-year-old sculptor who has lived heat-free in his Lower East Side loft for three decades, is bothered by such extremes. “Winter comes and goes,” he’ll tell you blithely, adjusting his black wool scarf and watch cap. (Along with fingerless gloves, long underwear and felt slippers, they are part of Mr. Ladda’s at-home uniform when the mercury dips.) "Unfurl