Function assembly_export_tree 

pub fn assembly_export_tree(
    document_name: &str,
    root_bodies: Vec<(String, BrepSolid)>,
    root_components: &[(String, String, Mat4)],
) -> Result<StepAssemblyExport, String>

Build the export tree for a document with components.

• document_name names the ROOT product.
• root_bodies is every resident solid of the document with its scene name; the component-owned ones are filtered out here, so the caller passes the scene exactly as it has it.
• root_components is one entry per ACOMP instance of the ROOT document: (component id, parts-library entry name, instance pose). The pose is the live one, so a solved or gizmo-moved instance exports where it sits.