ConstKnobs are plain display values in RoutingModuleConfig, at the paths their ids name.
layers, beforeLayerConfig, the waypoint images, icon mapping and label title, which take
structured, function or MapLibre values, and events aren't knobs. A displayUnits knob left
unset follows the global config.
Every routing knob with its kind, description, range or options and, where one value holds everywhere, its default. Route-wide knobs come first, then each drawn section type's. It's static data, readable with no map loaded.