feat: cross-node consumption edges over the mesh

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.

castle-api/src/castle_api/nodes.py

@@ -126,6 +126,9 @@ def mesh_deployments() -> dict:
                     "base_url": getattr(d, "base_url", None),
                     "subdomain": d.subdomain,
                     "endpoints": _endpoints_of_registry(d),
+                    "requires": [
+                        r.get("ref") for r in (getattr(d, "requires", None) or []) if r.get("ref")
+                    ],
                 }
             )
     return {"deployments": out}