The two-node bed proves a machine joins through the tunnel with the bus closed to it

A third machine makes its tunnel key, is issued a token for it, and
enrols while the anchor drops its packets to the bus at the first hook;
it can only have arrived over the tunnel (novox/hq ADR 0169).

scenarios/two-nodes.yml

@@ -28,6 +28,13 @@ machines:
     egress: true
     inbound: allow
     memory: 2GiB
+  # A third machine that joins through the tunnel with the bus closed to it (novox/hq ADR 0169). It
+  # carries none of the images: it only has to join.
+  joiner:
+    at: { segment: hosting, address: [192.0.2.30] }
+    egress: true
+    inbound: allow
+    images: []
 
 images:
   - mesh-controller:development