Function preview_add_manual_link 

pub fn preview_add_manual_link(
    source: &str,
    link: &ManualLinkConfig,
) -> Result<ManifestEdit, ManifestEditError>

Previews appending one exact manual relationship while preserving unrelated bytes.

Errors

Returns ManifestEditError when the declaration is invalid, duplicates an existing relationship, or the top-level manifest layout cannot be edited safely.