A first-class gap register

Layer: structure.

The competition explicitly wants "where the gaps are." Make what is not known a typed artifact (an open question with its blockers), not an implicit absence — otherwise gaps are invisible to retrieval.

A gap is distinct from a crux: a gap is what is not known (a neutral map-layer artifact); a crux is what would move the bottom line (verdict-relative, attached to an assessment). Both belong, in different layers.