Judge the machine's own networking beside what its modules run (hq ADR 0241)

A VPN client rewrote the laptop's resolver file and every mesh name failed
while each module read healthy: nothing asked the machine. The engine now
looks every 30 s at the resolver file the uplink holder declared (naming
the program that rewrote it), the names through each listed resolver
(NXDOMAIN for a mesh name's IPv6 address is a finding, issue 262), the
tunnel's handshake with the hub, the bus and the default route; a part is
unhealthy on its second failing look, and the statement carries it.

internal/link/queue.go

@@ -479,6 +479,15 @@ func declaredIn(body []byte) string {
 	return hex.EncodeToString(sum[:])
 }
 
+// Linked says whether a link to the bus is open now (novox/hq ADR 0241: the bus is one part of the
+// machine's networking its engine judges).
+func (q *Queue) Linked() bool {
+	q.init()
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	return q.bus != nil
+}
+
 // SayHealth says a health statement on the link open now (novox/hq ADR 0240, to-be 48 §4), and whether
 // the bus took it. **Not through the worker**: a statement is a word about the machine as it is, not an
 // account of an apply, and it must not wait behind one — a container crash-looping while a long apply