IStore
One level's storage, addressed by setting id (config-ui §CFGM-4).
In the namespace config. Declared in src/libs/libconfig/config/IStore.hpp.
Detailed description
An adapter over a store that already exists translates an id into the key shape its store speaks, and answers the one question the resolver needs: does this level set the setting? "Unset" is a state. A store that cannot report `has() == false` cannot take part, because the cascade rests on it.
A plain interface, not a QObject. The change signal is on the resolver.