Function build_aggregate_children_index 

pub fn build_aggregate_children_index<T>(
    content: &T,
    decoder: &mut EntityDecoder<'_>,
) -> FxHashMap<u32, Vec<u32>>
where
    T: AsRef<[u8]> + ?Sized,

Scan content for IfcRelAggregates and build the full (unfiltered) parent → children map used by propagate_voids_via_aggregates.