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/messages.go

@@ -238,6 +238,36 @@ type Health struct {
 	At time.Time `json:"at"`
 	// Resources are every long-running resource of a module this machine runs, by module and id.
 	Resources []ResourceHealth `json:"resources"`
+	// Network is the machine's own networking, judged by its engine (novox/hq ADR 0241): its resolver
+	// file, its names, its tunnel, its bus and its route. Absent from an engine older than that judging,
+	// which the controller reads as "not known", never as healthy.
+	Network *NetworkHealth `json:"network,omitempty"`
+}
+
+// NetworkHealth is the machine's networking in one statement (ADR 0241): the worst of its parts, since
+// when, and each part.
+type NetworkHealth struct {
+	State string        `json:"state"`
+	Since time.Time     `json:"since"`
+	Parts []NetworkPart `json:"parts"`
+}
+
+// NetworkPart is one part of a machine's networking, as its engine judged it on its second look.
+type NetworkPart struct {
+	// Part is resolv-conf, names, tunnel, bus or route.
+	Part  string `json:"part"`
+	State string `json:"state"`
+	// Reason is why it is not healthy, in words with no address, path or domain; Said the detail.
+	Reason string `json:"reason,omitempty"`
+	Said   string `json:"said,omitempty"`
+	// Writer is the program that rewrote the resolver file, when it can be named; Owner the module whose
+	// file it is — the uplink's holder.
+	Writer string `json:"writer,omitempty"`
+	Owner  string `json:"owner,omitempty"`
+	// Toward is what the failure points at: "hub", or each resolver's address that failed.
+	Toward []string  `json:"toward,omitempty"`
+	Since  time.Time `json:"since"`
+	Streak int       `json:"streak,omitempty"`
 }
 
 // The states a long-running resource is said in (ADR 0240 §4).

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