ConstKnobs are plain display values in POIsModuleConfig, at the paths their ids name. They set
whether POIs show, which categories, and their look (size, zoom, label colours, micro
markers). They restyle the style's own POI layers and read the style's value while unset.
icon.categoryIcons, which takes images, filters.ids, which takes the ids of single POIs, and
events aren't knobs.
Every POIs knob with its kind, description, options and, where one value holds everywhere, its default. It's static data, readable with no map loaded.