Function install_child_fixture 

pub fn install_child_fixture(base: &Path) -> RuntimePaths

Build a REAL child root under base and make it this process’s child root. Nothing is bypassed: the overlay’s own checks run over it.

Written exactly as the user daemon and supervisor will write them: a user key (seed [7; 32]) that signs the project certificate (ProjectCert::sign) and an empty-rules parent policy (parent_policy::export_rules_to), the user.pub pin (write_user_pin), and a 0600 project.key matching the certificate. The version pin is written by Prepared::commit, as on a real boot.