Function collect_id_spans 

pub fn collect_id_spans(
    val: &JsonLdVal,
    resolver: &ContextResolver,
    out: &mut HashMap<String, Range<usize>>,
)

Walk a JsonLdVal tree and collect the byte spans of every @id value, keyed by the expanded IRI (resolved via resolver).

A span entry maps expanded_iri → byte_range_of_@id_value_in_source.