Zurich is a city of polite reserve, solid prosperity and hard work. ''Zurich is so clean,'' said James Joyce, ''that should you spill your soup on the Bahnhofstrasse, you could eat it up without a spoon.'' There are many symbols of Zurich's dependable character - the Bahnhofstrasse banks, the labeled shrubs on nature trails - but few so apt as the Hotel Baur au Lac.