Take over the found tunnel: its key, its port, its peers; stop it, never flush

On an adopted machine the private network takes the predecessor's tunnel
over in place (hq ADR 0105). Genesis finds the one interface up besides the
mesh's own, settles the hub's port and the mesh's range on it, and skips
ADR 0100's non-overlap check for a range that is now the tunnel's; a
--hub-port or --overlay-range that disagrees is refused naming the tunnel's.

At enrolment the found interface's private key becomes this node's overlay
key — the one credential the mesh takes rather than mints — stored where a
generated one is stored, never printed and never sent; the tunnel (port,
address, range, peers) travels with the keys so the mesh composes from it
before the first declaration.

The interface's service may say what it takes over. Before the mesh's unit
starts, the found configuration is kept like any held file and the found
unit is stopped and disabled; nothing is flushed, and an interface still up
after its unit stopped refuses the takeover rather than half-working. The
report says what was carried: interface, port, range, peer count, taken or
not, and where the original was kept.

internal/apply/hold_test.go

@@ -19,6 +19,8 @@ import (
 type machine struct {
 	containers map[string]*fakeContainer
 	asked      []string
+	// wgUp is what `wg show interfaces` answers: the tunnels up on the machine.
+	wgUp string
 
 	// units are service units by name, as systemd would report them; volumes are the runtime's
 	// named volumes.
@@ -94,6 +96,9 @@ func (m *machine) run(_ context.Context, name string, args ...string) (string, e
 	if name == "systemctl" {
 		return m.systemctl(args)
 	}
+	if name == "wg" {
+		return m.wgUp, nil
+	}
 	if name == "getent" {
 		if m.users[args[len(args)-1]] {
 			return args[len(args)-1] + ":x:1500:1500::/home/" + args[len(args)-1] + ":/bin/bash\n", nil