Check-coupling graph

Which content surfaces each gates.list gate binds together. Source of truth is the per-gate graph: manifests; this is their projection, regenerated by bash gate-sdk/bin/run-gates.sh --emit graph and drift-checked by assertion E.

Nodes are surfaces; an edge is a gate. A bidirectional edge is a coupling bijection; a self-loop guards one surface. Amber edges are cycle valves (valve=PROPOSED) - where a leading surface may run ahead via a queue-tracked marker.