Function install_derived 

pub fn install_derived<T>(linker: &mut Linker<'_>) -> Result<()>
where
    T: CitizenRuntime,

Installs a derived citizen T as a class value in linker.

Registers a class backing T’s callable, class, and read-constructor contracts and binds it under T::citizen_symbol. This is the call a citizen’s generated InstallFn makes.