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/.

internal/overlay/graph.go

@@ -26,6 +26,40 @@ type Node struct {
 	Site     string
 	Hub      bool
 	Address  string
+
+	// Carried are the peers of the tunnel this node took over (novox/hq ADR 0105): machines the
+	// mesh has no record of, each known by the public key and the address the found tunnel routed
+	// to it. Only a hub has any. They stay in its peer list until a node enrols with that key —
+	// from then on the node is the peer.
+	Carried []Carried
+	// TakesOver names the found tunnel this node's private network replaces: its unit is stopped
+	// and disabled, never flushed, and its configuration kept, before the mesh's interface comes
+	// up with the found key. Nil on a node that raises the mesh's interface beside whatever it has.
+	TakesOver *TakeOver
+}
+
+// Carried is one peer of an adopted tunnel that has not enrolled: a peer of the tunnel, not a
+// node of the mesh.
+type Carried struct {
+	Key     string
+	Address string
+}
+
+// TakeOver is the found tunnel a node's private network takes over, as the host is told it.
+type TakeOver struct {
+	Interface string
+	Unit      string
+	Config    string
+}
+
+// CarriedName is how a carried peer is named in a peer list: it has no node name, so it is named
+// by the key its packets arrive under.
+func CarriedName(key string) string {
+	short := key
+	if len(short) > 8 {
+		short = short[:8] + "…"
+	}
+	return "a peer of the tunnel (" + short + ")"
 }
 
 // Reachable reports whether other nodes can dial this one. Declared, never inferred.
@@ -145,7 +179,9 @@ func Compute(nodes []Node, overlayCIDR string) (Graph, error) {
 			// The hub holds every node that does not share a site with it, because those nodes
 			// route through it and it must know where to send the replies. Ones it cannot dial
 			// will dial it.
+			enrolled := map[string]bool{}
 			for _, other := range usable {
+				enrolled[other.Key] = true
 				if other.Name == self.Name || (self.Site != "" && self.Site == other.Site) {
 					continue
 				}
@@ -156,6 +192,21 @@ func Compute(nodes []Node, overlayCIDR string) (Graph, error) {
 					Why:      "routes through this hub",
 				})
 			}
+			// And every peer of the tunnel it took over that has not enrolled (novox/hq ADR
+			// 0105): the same key and the same address the found tunnel had for it, so the
+			// machine behind it cannot tell the tunnel changed hands. No endpoint — it dials in,
+			// as it always did. Once a node enrols with that key, the node's entry above is the
+			// peer, and WireGuard takes one entry per key.
+			for _, c := range self.Carried {
+				if enrolled[c.Key] {
+					continue
+				}
+				peers = append(peers, Peer{
+					Name: CarriedName(c.Key), Key: c.Key,
+					Allowed: c.Address + "/32",
+					Why:     "carried from the tunnel this hub took over — a peer of the tunnel, not yet a node of the mesh",
+				})
+			}
 		}
 
 		sort.Slice(peers, func(i, j int) bool { return peers[i].Name < peers[j].Name })