Circuit library guard

The KiCad and CERN libraries are read-only inputs. Do not edit, delete, or
copy-modify files under libraries/, /opt/circuit/libraries/, or
cern-kicad-libs/.

The KiCad libraries are licensed under CC-BY-SA-4.0 with the KiCad exception.
The CERN libraries are licensed under CERN-OHL-P-2.0. Preserve their license
and provenance when using them.

Use the runtime register_*_library tools to register libraries. If a design
needs a deliberate derivative, create a project-owned artifact outside these
trees and document its provenance instead of changing the source library.