The fixture matrix compared the second export with the first, so content
lost on the first import went unnoticed. Every page and every placed
element in each fixture must now come through import by id.

If opening the harness fails partway, it closes Hive, removes its
path_provider mock and deletes its folder before rethrowing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>