[client] Unify route selection in the route manager

Move route select/deselect handling from the daemon server into exported
routemanager methods (SelectRoutes, DeselectRoutes, SelectAllRoutes,
DeselectAllRoutes) so every consumer shares one implementation: v4/v6
exit-pair expansion, exit-node mutual exclusion, and selection triggering.

Previously the exit-node exclusivity lived only in the daemon's
SelectNetworks RPC, so the Android and iOS bindings could leave two exit
nodes selected until the next network map reconciliation. Both bindings
now call the shared manager methods and enforce exclusivity at toggle
time, matching the desktop behavior.

client/internal/routemanager/manager.go

@@ -52,6 +52,10 @@ type Manager interface {
 	UpdateRoutes(updateSerial uint64, serverRoutes map[route.ID]*route.Route, clientRoutes route.HAMap, useNewDNSRoute bool) error
 	ClassifyRoutes(newRoutes []*route.Route) (map[route.ID]*route.Route, route.HAMap)
 	TriggerSelection(route.HAMap)
+	SelectRoutes(ids []route.NetID, appendRoute bool) error
+	DeselectRoutes(ids []route.NetID) error
+	SelectAllRoutes()
+	DeselectAllRoutes()
 	GetRouteSelector() *routeselector.RouteSelector
 	GetClientRoutes() route.HAMap
 	GetSelectedClientRoutes() route.HAMap
@@ -800,7 +804,7 @@ func (m *DefaultManager) collectExitNodeInfo(clientRoutes route.HAMap) exitNodeI
 	var info exitNodeInfo
 
 	for haID, routes := range clientRoutes {
-		if !m.isExitNodeRoute(routes) {
+		if !isExitNodeRoutes(routes) {
 			continue
 		}
 
@@ -820,13 +824,6 @@ func (m *DefaultManager) collectExitNodeInfo(clientRoutes route.HAMap) exitNodeI
 	return info
 }
 
-func (m *DefaultManager) isExitNodeRoute(routes []*route.Route) bool {
-	if len(routes) == 0 {
-		return false
-	}
-	return route.IsV4DefaultRoute(routes[0].Network) || route.IsV6DefaultRoute(routes[0].Network)
-}
-
 func (m *DefaultManager) categorizeUserSelection(netID route.NetID, info *exitNodeInfo) {
 	if m.routeSelector.IsSelected(netID) {
 		info.userSelected = append(info.userSelected, netID)