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.

core/src/castle_core/registry.py

@@ -86,6 +86,10 @@ class Deployment:
     # Declared desired state (from the deployment's `enabled:`). `castle apply`
     # activates enabled deployments and deactivates disabled ones. Default True.
     enabled: bool = True
+    # Deployment `requires` (list of {kind, ref, bind}) — carried so the mesh can
+    # draw cross-node consumption. A remote consumer's ref resolves against the
+    # provider set across nodes.
+    requires: list[dict] = field(default_factory=list)
 
 
 @dataclass