Function declare 

pub fn declare(
    root: &Path,
    id: &str,
    title: &str,
    producer: &str,
) -> Result<PathBuf, Error>

What every report writes beside its rendering: the declaration that makes the directory a surface.

The producer states the intent — its id, where it belongs, what built it — and discovery reads it back. Nothing else in the repository is told that the surface exists.