Function ingest_rdf_xml 

pub fn ingest_rdf_xml(
    input: &Path,
    output: &Path,
) -> Result<IngestStats, Box<dyn Error>>

Ingest an RDF/XML file at input — same output format as ingest_ntriples.

Streams triples into ExternalSorter so peak RAM stays bounded at ~48 MB regardless of input size.