mesh: carry gateway_domain so peers can launch remote apps

The mesh payload didn't include a node's acme domain, so a peer had no way
to build launch URLs for another machine's exposed apps. Publish
gateway_domain in the MQTT registry payload and surface it per-deployment on
/mesh/deployments as `domain`. The dashboard palette and System Map now build
`https://<subdomain>.<domain>` for remote http-exposed apps (references use
their base_url), making primer's apps launchable from civil.

castle-api/src/castle_api/mqtt_client.py

@@ -37,6 +37,9 @@ def _registry_to_json(registry: NodeRegistry) -> str:
         "node": {
             "hostname": registry.node.hostname,
             "gateway_port": registry.node.gateway_port,
+            # acme domain — lets peers build launch URLs (<subdomain>.<gateway_domain>)
+            # for this node's exposed apps. Omitted when the node has no domain.
+            "gateway_domain": registry.node.gateway_domain,
         },
         "deployed": {},
     }
@@ -83,6 +86,7 @@ def _json_to_registry(payload: str) -> NodeRegistry:
         hostname=node_data.get("hostname", ""),
         castle_root=node_data.get("castle_root"),
         gateway_port=node_data.get("gateway_port", 9000),
+        gateway_domain=node_data.get("gateway_domain"),
     )
     deployed: dict[str, Deployment] = {}
     for key, comp_data in data.get("deployed", {}).items():

castle-api/src/castle_api/nodes.py

@@ -115,16 +115,18 @@ def _endpoints_of_registry(d: object) -> list[dict]:
 def mesh_deployments() -> dict:
     """Flattened remote (mesh-discovered) deployments with derived endpoints — the
     data the System Map needs to render other machines. Local node excluded (it's
-    already in /graph). Cross-node dependency *edges* need `requires` in the mesh
-    payload, which the registry doesn't carry yet."""
+    already in /graph). Each entry carries its node's `domain` (gateway acme domain)
+    so peers can build launch URLs `<subdomain>.<domain>` for exposed apps."""
     out: list[dict] = []
     for hostname, remote in mesh_state.all_nodes(include_stale=True).items():
+        domain = getattr(remote.registry.node, "gateway_domain", None)
         for _kind, name, d in remote.registry.all():
             out.append(
                 {
                     "name": name,
                     "kind": d.kind,
                     "node": hostname,
+                    "domain": domain,
                     "port": d.port,
                     "base_url": getattr(d, "base_url", None),
                     "subdomain": d.subdomain,