Strolling along the twisted streets lined by aged buildings in a jumble of architectural styles, my first thought was that the setting would be a city planner’s nightmare.   Then my mind wandered to the rich history that has unfolded in the often-overlooked neighborhood in one of the most-visited cities in the world. Throughout the Marais district of Paris, crumbling mansions that cling to memories of past glory days are neighbors to trendy shops and galleries. Bustling restaurants abut small, offbeat museums.