Function nodes_of_kind 

fn nodes_of_kind(document: &Document, kind: NodeKind) -> Vec<NodeId>

Collect the ids of every node of kind in document order. nodes() iterates the arena by ascending [mos_core::NodeId] (allocation order), and the lowerer allocates nodes in source order, so the result is stable document order regardless of nesting depth. Shared by figure numbering and section_order so both passes agree on what “document order” means.