Adopt the predecessor's tunnel in place: its range, its address, its peers

On an adopted hub the private network takes over the tunnel it finds rather
than running beside it (hq ADR 0105): two tunnels leave the mesh's unreachable
through the provider's filter, so no machine can ever join.

The node presents the found tunnel when it enrols, under the key it took as
its own; the inventory records it (node.tunnel, tunnel_peer — migration 0031)
and the mesh composes from it: the overlay's range is the adopted tunnel's,
the hub is placed at the tunnel's address on the tunnel's port, and every
peer the tunnel had is carried in the hub's peer list as a peer of the
tunnel, not a node of the mesh, until a node enrols with that key — which
then keeps the address the tunnel had for it. A fresh node never gets an
address the tunnel holds. The hub's declaration tells the host which unit to
take over; the host's account of carrying it is recorded and shown.

Every reader of the range follows the setting; nothing stores it. A found
tunnel under another key is recorded and not adopted, so ADR 0100's
non-overlap rule keeps applying where a tunnel is left running beside the
mesh's. A lab bed and test skeleton for "How it is checked" are under lab/.

cmd/mesh-controller/adoption.go

@@ -45,6 +45,9 @@ func showMode(ctx context.Context, inv *inventory.Inventory, node inventory.Node
 		return nil
 	}
 	fmt.Printf("  firewall found    %s\n", orNone(said.Firewall))
+	if err := showTunnel(ctx, inv, node.Name); err != nil {
+		return err
+	}
 	if len(said.Held) == 0 {
 		fmt.Printf("  holding           nothing found\n")
 	}
@@ -63,6 +66,37 @@ func showMode(ctx context.Context, inv *inventory.Inventory, node inventory.Node
 	return nil
 }
 
+// showTunnel is the node show lines about the tunnel an adopted node found and carried (novox/hq
+// ADR 0105): what it presented at enrolment, and what it last said about taking it over.
+func showTunnel(ctx context.Context, inv *inventory.Inventory, name string) error {
+	tunnel, err := inv.TunnelOf(ctx, name)
+	if errors.Is(err, inventory.ErrNoTunnel) {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	fmt.Printf("  tunnel found      %s on port %d, %s in %s, %d peer(s)\n",
+		tunnel.Interface, tunnel.Port, tunnel.Address, tunnel.Range, len(tunnel.Peers))
+	carried, said, err := inv.CarriedTunnelOf(ctx, name)
+	if err != nil {
+		return err
+	}
+	switch {
+	case !said:
+		fmt.Printf("  %-17s not yet taken over — the node has not said so\n", "")
+	case carried.Taken:
+		fmt.Printf("  %-17s taken over: %s is down and disabled, never flushed; the mesh's interface "+
+			"runs with its key, port and %d peer(s)\n", "", carried.Interface, carried.Peers)
+	default:
+		fmt.Printf("  %-17s NOT taken over: %s is still the interface the peers reach\n", "", carried.Interface)
+	}
+	if said && carried.Kept != "" {
+		fmt.Printf("  %-17s its configuration's original kept at %s\n", "", carried.Kept)
+	}
+	return nil
+}
+
 func orNone(s string) string {
 	if s == "" {
 		return "none reported"