Soho is Manhattan’s most densely packed retail district — as reflected both in terms of numbers of shoppers and the fewest vacant stores. But Madison Avenue above East 59th Street remains the gold standard for its concentration of global brands and the affluence of customers, most of whom live in the neighborhood — unlike Soho’s tourists and day-trippers. So we asked Matthew Bauer, head of the Madison Avenue Business Improvement District, to fill us in on the whirlwind changes we noticed