Pin accepted GNN 4.0.0 main for reciprocal interchange

[docxology]

GNN 4.0.0 now has accepted main 1bc3a76eccccb2cc5ce8601770714075b3ec48db, integrated normally through GNN PR #251 after all 18 exact-head checks passed. Update only .github/gnn-pair.json from 7983a715af62b4911083fb44677cf9fe8d615d33 to that reviewed main, following the canonical pair-pin bump procedure.

The three GNN exporters and their two contract documents are byte-identical between those revisions. The current GEO consumers retain gnn-geo-infer/1, gnn-geo-infer/2, and gnn-geo-infer/factored/1. No consumer, model, training, dependency, workflow, or documentation source changes are included.

Validation at 6736f19dada8a86b9d0d292c89f8081873fe802d: all 21 current checks pass, including paired interchange on Python 3.11/3.12, four Linux/Windows import probes, and all applicable repository CI. Exact current CI. Downloaded paired receipts independently bind clean GEO/GNN checkouts at the named revisions and record deterministic finite numerical replay. The pin and unchanged contracts received independent review. No new scheduled slow-lane execution is claimed.

The reciprocal pair establishes the named interchange/custody boundary; it does not claim universal scientific, hardware, or formal acceptance. GNN publication remains pending this normal companion integration, main verification and final release checks.