Function propagate_voids_to_parts 

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

Propagate void (opening) relationships from aggregate parents to their children and return a child-part → parent-element map covering every emitted aggregate IfcWall → IfcBuildingElementPart pair.

In IFC, multilayer walls use IfcRelAggregates to decompose a parent IfcWall into child IfcBuildingElementPart entities (one per material layer). The IfcRelVoidsElement relationships reference the parent wall, but the individual layer parts also need void subtraction to cut windows/doors through each layer.

This function scans for IfcRelAggregates and, in the same pass:

1. Copies the host’s void relationships to EVERY aggregated descendant, any depth and any type, whether or not it has a Representation (propagate_voids_via_aggregates; an entry for a geometry-less part is never looked up). Each layer slice still gets its window/door cutouts.
2. Returns a [FxHashMap] mapping every IfcBuildingElementPart that has its own Representation to its NEAREST aggregating ancestor that also has one. Callers use this to skip per-part geometry emission when the “merge multilayer wall as a single solid” toggle is on (issue #540): the ancestor’s merged solid is drawn instead, which is only possible when that ancestor has geometry of its own.

The map walk descends to the same depth as the void propagation, but only through representation-less intermediates (an IfcElementAssembly that merely groups), so IfcWall -> IfcElementAssembly -> IfcBuildingElementPart lands the part on the wall. A child with its own Representation ends the descent: it is the parent of its own subtree in this same loop (tests/void_index_nested_aggregates.rs). Parents without their own geometry never appear as a map value, so the caller can never “skip” the only geometry available for the assembly.