Carry each deployment's `requires` (deployment refs, no secrets) from config →
registry → the mesh MQTT payload, and expose it on /mesh/deployments. The System
Map resolves a remote deployment's ref against the provider set across nodes
(same machine → local provider → other machine) and draws a dashed cross-node
edge. This is the multi-node payoff: e.g. primer's castle-api → civil's mqtt.