Function export_step_assembly_report 

pub fn export_step_assembly_report(
    assembly: &StepAssemblyExport,
    unit: &str,
    timestamp: &str,
    pmi: Option<&StepPmi<'_>>,
) -> Result<StepExportReport, String>

Write a product structure as an AP242 Part 21 document: one product per distinct part, one NEXT_ASSEMBLY_USAGE_OCCURRENCE per placement, exact B-rep geometry in each part’s own frame.

The file is named after the ROOT product, and the optional PMI block rides on the root exactly as it does in the flat lane — with each shape aspect attached to the product that actually OWNS the geometry it names.