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/mqtt_client.py

@@ -67,6 +67,9 @@ def _registry_to_json(registry: NodeRegistry) -> str:
             entry["tcp_port"] = comp.tcp_port
         if getattr(comp, "base_url", None):
             entry["base_url"] = comp.base_url
+        # requires — deployment refs (no secrets), so peers can draw cross-node deps.
+        if getattr(comp, "requires", None):
+            entry["requires"] = comp.requires
         data["deployed"][NodeRegistry.key(comp.kind, name)] = entry
 
     return json.dumps(data)
@@ -101,6 +104,7 @@ def _json_to_registry(payload: str) -> NodeRegistry:
             managed=comp_data.get("managed", False),
             tcp_port=comp_data.get("tcp_port"),
             base_url=comp_data.get("base_url"),
+            requires=comp_data.get("requires", []),
         )
     return NodeRegistry(node=node, deployed=deployed)
 

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}