Fully match Python fsSelection handling

https://github.com/googlefonts/fontmake-rs/pull/298#discussion_r1205332559 notes that we do not completely match:

> glyphsLib contains additional logic, see build_stylemap_names (that is called when exporting glyphs->ufo by fontmake because minimize_glyphs_diffs is False when compiling):
>
> https://github.com/googlefonts/glyphsLib/blob/42bc1db912fd4b66f130fb3bdc63a0c1e774eb38/Lib/glyphsLib/builder/names.py#L27-L43
>
> the final ufo.info.styleMapStyleName in the end is what matters to decide the style flags for ufo2ft, and the default UFO master that glyphsLib produces is going to be the default master TTF which varLib will duplicate to use as basis for the new variable font.

Start with the linked pull-request discussion and the referenced glyphsLib builder/names.py build_stylemap_names implementation. Compare fontc's fsSelection handling with that logic, focusing on the resulting ufo.info.styleMapStyleName; done means the Rust implementation fully matches the referenced Python behavior.

Written by the indexing model from the issue text.