state: add HA health prober

Ping HA subnet routers each probe cycle and mark unresponsive nodes
unhealthy. Reconnecting a node clears its unhealthy state since the
fresh Noise session proves basic connectivity.

Updates #2129
Updates #2902

hscontrol/state/state.go

@@ -574,6 +574,10 @@ func (s *State) Connect(id types.NodeID) ([]change.Change, uint64) {
 
 	log.Info().EmbedObject(node).Msg("node connected")
 
+	// Reconnecting clears any prior unhealthy state — the node proved
+	// basic connectivity by establishing the Noise session.
+	s.primaryRoutes.ClearUnhealthy(id)
+
 	// Use the node's current routes for primary route update.
 	// AllApprovedRoutes() returns only the intersection of announced and approved routes.
 	routeChange := s.primaryRoutes.SetRoutes(id, node.AllApprovedRoutes()...)
@@ -1184,6 +1188,11 @@ func (s *State) RoutesForPeer(
 	return reduced
 }
 
+// PrimaryRoutes returns the primary routes tracker.
+func (s *State) PrimaryRoutes() *routes.PrimaryRoutes {
+	return s.primaryRoutes
+}
+
 // PrimaryRoutesString returns a string representation of all primary routes.
 func (s *State) PrimaryRoutesString() string {
 	return s.primaryRoutes.String()