Function write_dataset_with_metadata 

pub fn write_dataset_with_metadata(
    dict: &Dictionary,
    default_index: &GraphIndex,
    named: &[(String, GraphIndex)],
    has_quads: bool,
    pyramid_meta: &[u8],
    pyramid_levels: u16,
    metadata: &[u8],
    text_index: &[u8],
) -> Vec<u8> 

Serialize a dataset with an opaque metadata payload occupying the file’s metadata section (the application layer defines its meaning — the CLI stores a JSON Dataset Card there). The section sits immediately after the header and before the dictionary, so metadata_offset stays at HEADER_LEN and every downstream section shifts by metadata.len(). The payload is folded into the content_hash, so verify covers it and it is tamper-evident.

Passing an empty metadata is byte-identical to write_dataset: the section is omitted (metadata_len = 0, dictionary_offset = HEADER_LEN) and the hash is computed over exactly the same parts (a zero-length hash update is a no-op).