Function export_step_handles 

pub fn export_step_handles(
    handles: &[u32],
    name: &str,
    unit: &str,
    timestamp: &str,
) -> Result<String, String>

Native: export the CURRENT resident solids (by handle) to an ISO-10303-21 STEP document. Reads each resident solid out of the thread-local registry — the SAME registry display_payload_handle_native reads — clones them into a Vec<BrepSolid>, and hands the batch to export_step. The engine-native app’s Export→STEP lane calls this with the handles the pipeline left resident after the last history run, so the topology never crosses a boundary as JSON. String error (JsValue-free — it links + runs on native and wasm alike).