Function export_step_assembly_handles 

pub fn export_step_assembly_handles(
    document_name: &str,
    named: &[(String, u32)],
    components: &[(String, String, Mat4)],
    unit: &str,
    timestamp: &str,
    pmi: Option<&StepPmi<'_>>,
) -> Result<StepExportReport, String>

STRUCTURED export: the resident scene PLUS the document’s assembly model, as an AP242 file with real product structure — one PRODUCT per parts-library entry, one NEXT_ASSEMBLY_USAGE_OCCURRENCE per placed instance.

named is the whole resident scene (component-owned bodies included — the tree builder filters them, because their geometry is written unposed in the part’s own product instead). components is one entry per ACOMP instance of the ROOT document — (component id, parts-library entry name, live pose) — which only the caller knows, since a solved or gizmo-moved instance’s pose lives in the app’s projection of the scene.