A node in an %Artefact{} graph.
id is the local graph identifier (e.g. "n0"), stable within one artefact.
uuid is the global identity — a UUIDv7 that survives compose and harmonise.
labels are semantic type tags (base_label is applied at output time, not stored here).
position is an optional %{x, y} hint for visual layout, sourced from Arrows JSON.