Adopt the tunnel as a mesh fact, refuse a mismatched takeover, and rekey after enrolment

Review of the ADR 0105 build (hq ADR 0105). Four things it got wrong and one
path it lacked:

- A predecessor spoke's tunnel names one peer, the hub, routed the whole
  range; recording refused it and the whole enrolment failed. Range-routed
  peers are skipped now — only the hub's peers are ever carried.
- The range and the carried peers were conditions on the node being adopted,
  so converging the hub would have renumbered the mesh and dropped the peers
  still reaching it. They are facts of the tunnel record now, mode aside; the
  takeover alone is declared to an adopted node. Converging the hub is refused
  while a carried peer has not enrolled, naming it.
- A push composed a takeover for a hub whose address or endpoint disagreed
  with the tunnel, which would have the host stop the found interface and
  raise the mesh's where no peer listens. The graph refuses to compose it,
  naming both and the placement that fixes it.
- The host's account said taken or not; "found down and the mesh's not up"
  read as not taken. Three states now, and an account on every takeover.
- A hub that enrolled before this feature holds a key of its own, and
  re-enrolling would rotate every key the mesh sealed credentials to. A node
  now rekeys in a report, signed with its identity key over the key it
  leaves, the key it takes and the tunnel; the mesh verifies against the live
  key, refuses a stale or foreign proof, records key and tunnel, and moves a
  hub to the tunnel's address. `overlay show` names the path for a hub that
  found no tunnel.

Also: a carried IPv6 peer is routed /128, and identity.ForTest exists so the
link can be tested against a real identity store.

cmd/mesh-controller/adoption.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/novox/mesh-controller/internal/catalogue"
 	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/overlay"
 )
 
 // A node is adopted or converged (novox/hq ADR 0100), and it is said to be adopted wherever the
@@ -85,12 +86,19 @@ func showTunnel(ctx context.Context, inv *inventory.Inventory, name string) erro
 	switch {
 	case !said:
 		fmt.Printf("  %-17s not yet taken over — the node has not said so\n", "")
-	case carried.Taken:
+	case carried.State == inventory.CarriedTaken:
 		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)
+	case carried.State == inventory.CarriedDown:
+		fmt.Printf("  %-17s TUNNEL DOWN: %s is stopped and the mesh's interface is not up — the peers "+
+			"reach nothing. On the machine: systemctl start %s\n", "", carried.Interface,
+			"wg-quick@"+carried.Interface)
 	default:
 		fmt.Printf("  %-17s NOT taken over: %s is still the interface the peers reach\n", "", carried.Interface)
 	}
+	if said && carried.Note != "" {
+		fmt.Printf("  %-17s %s\n", "", carried.Note)
+	}
 	if said && carried.Kept != "" {
 		fmt.Printf("  %-17s its configuration's original kept at %s\n", "", carried.Kept)
 	}
@@ -247,6 +255,28 @@ func converge(ctx context.Context, open *stores, node string, yes bool, digest s
 		return "", fmt.Errorf("%s still holds what it found, and a service is taken on its own, "+
 			"never by the flip:\n%s", node, strings.Join(holding, "\n"))
 	}
+	// And refused while a peer of the tunnel this hub took over has not enrolled (novox/hq ADR
+	// 0105): the flip loads the derived filter and retires the found firewall, and a machine the
+	// mesh has no record of is not one the filter admits — it would go dark.
+	if _, hubName, adopted, err := inv.AdoptedTunnel(ctx); err != nil {
+		return "", err
+	} else if adopted && hubName == node {
+		carried, err := inv.CarriedPeers(ctx)
+		if err != nil {
+			return "", err
+		}
+		var waiting []string
+		for _, c := range carried {
+			if c.EnrolledAs == "" {
+				waiting = append(waiting, fmt.Sprintf("  %s at %s", overlay.CarriedName(c.PublicKey), c.Address))
+			}
+		}
+		if len(waiting) > 0 {
+			return "", fmt.Errorf("%s carries peers of the tunnel it took over that have not enrolled, and "+
+				"converging would cut them off — enrol each first (`overlay show` says which are enrolled):\n%s",
+				node, strings.Join(waiting, "\n"))
+		}
+	}
 
 	shelf, err := inv.Catalogue(ctx)
 	if err != nil {