Fix broken test files

Reveals by trying to open the new golden test files from !1183 (closed) in KiCad. Three issues:

• One, a mistake in the dev-docs: kicad/services/kicad-dev-docs!71 (merged)
• Two, a typo in the serialisation resulting in broken s-expr syntax (but this code path is probably never used in the footprints, as it's zone filling)
• A test that wrote a string into the width field. Add type safety in the underlying class.

Suggest not to squash as the commits are each self-contained.