Boudin is classic San Francisco, where a show kitchen pumps warm air scented with the tang of fresh sourdough into oncoming tourist traffic and funnels hungry travelers straight into a crazy money trap full of gourmet novelties. The soup, however, is easy to love, redolent of briney clams, balanced by just so cubed potatoes, all swimming in a base of mild herbs and cream that is whipped up to a strangely fluffy consistency. Texture is what sets Boudin apart, because the base is somewhere between a finely mashed potato, hearty enough to stand a fork in, and frothy clouds.