Function flat_voids 

pub fn flat_voids(
    void_index: &FxHashMap<u32, Vec<u32>>,
) -> (Vec<u32>, Vec<u32>, Vec<u32>)

Flat wire encoding of the void index: (keys, counts, values) in the shape processGeometryBatch accepts.

Emitted sorted by host id (u32 ascending). FxHashMap iteration order is seed-free and therefore stable today, but it is an implicit insertion+hash-order artifact; the sort makes the wire byte order an explicit contract (pinned by the mesh-output determinism manifest, docs/architecture/mesh-determinism.md). Consumers rebuild a map from the flat arrays (processGeometryBatch), so they are order-insensitive.