[client] mirror v4 exit selection onto v6 pair at write time

The synthesized "-v6" exit route shares its v4 base's NetID plus a "-v6"
suffix. Selection state was reconciled at read time via effectiveNetID, a
mirror that could only be applied on exit-node code paths, which forced a
parallel IsSelectedForExitNode() alongside IsSelected() and a clearPairedV6Locked()
orphan cleanup on every toggle. That machinery still missed the case observed
in the field: a persisted state with the v4 base deselected but its "-v6"
sibling explicitly selected (orphaned). Because effectiveNetID returns the v6
entry itself once it carries explicit state, and clearPairedV6Locked only fires
on a live toggle, the loaded orphan survived and the ::/0 route leaked onto the
tunnel despite the exit node being disabled, breaking IPv6 (happy eyeballs).

Treat the v4/v6 exit pair as a single toggle and keep state consistent at write
time instead. RouteSelector.SyncPairedSelection forces the "-v6" entry to match
its v4 base unconditionally, resetting any orphaned explicit state. The route
manager, which knows the route prefixes, computes the pairs (V6ExitMergeSet) and
calls it from updateRouteSelectorFromManagement before selection is read, so both
collectExitNodeInfo and FilterSelectedExitNodes see consistent state, including
pairs loaded from persisted selector state.

This removes effectiveNetID, IsSelectedForExitNode and clearPairedV6Locked; the
selector is literal again and no longer needs the "exit-node paths only" caveat.
HasUserSelectionForRoute and applyExitNodeFilter use the raw NetID.

Adds a selector test for SyncPairedSelection (including the orphaned-v6 case) and
a route-manager test reproducing the persisted-orphan scenario from the field log.

client/internal/routemanager/manager.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"runtime"
 	"slices"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -700,6 +701,8 @@ func resolveURLsToIPs(urls []string) []net.IP {
 
 // updateRouteSelectorFromManagement updates the route selector based on the isSelected status from the management server
 func (m *DefaultManager) updateRouteSelectorFromManagement(clientRoutes route.HAMap) {
+	m.mirrorV6ExitPairSelections(clientRoutes)
+
 	exitNodeInfo := m.collectExitNodeInfo(clientRoutes)
 	if len(exitNodeInfo.allIDs) == 0 {
 		return
@@ -709,6 +712,24 @@ func (m *DefaultManager) updateRouteSelectorFromManagement(clientRoutes route.HA
 	m.logExitNodeUpdate(exitNodeInfo)
 }
 
+// mirrorV6ExitPairSelections keeps every synthesized "-v6" exit route's selection
+// consistent with its v4 base. The v4/v6 exit pair is a single toggle, so the v6
+// entry always follows the base: deselecting the v4 exit node also drops its ::/0
+// pair, and any stale (orphaned) explicit selection on the v6 entry is reset. This
+// runs before selection is read so both collectExitNodeInfo and FilterSelectedExitNodes
+// see consistent state, including pairs loaded from persisted selector state.
+func (m *DefaultManager) mirrorV6ExitPairSelections(clientRoutes route.HAMap) {
+	routesByNetID := make(map[route.NetID][]*route.Route, len(clientRoutes))
+	for haID, routes := range clientRoutes {
+		routesByNetID[haID.NetID()] = routes
+	}
+
+	for v6ID := range route.V6ExitMergeSet(routesByNetID) {
+		baseID := route.NetID(strings.TrimSuffix(string(v6ID), route.V6ExitSuffix))
+		m.routeSelector.SyncPairedSelection(baseID, v6ID)
+	}
+}
+
 type exitNodeInfo struct {
 	allIDs               []route.NetID
 	selectedByManagement []route.NetID
@@ -745,10 +766,7 @@ func (m *DefaultManager) isExitNodeRoute(routes []*route.Route) bool {
 }
 
 func (m *DefaultManager) categorizeUserSelection(netID route.NetID, info *exitNodeInfo) {
-	// Use the exit-node-aware check so a synthesized "-v6" route inherits the v4
-	// base's selection: deselecting the v4 exit node must also drop its ::/0 pair,
-	// otherwise the v6 default route leaks into the tunnel despite the deselect.
-	if m.routeSelector.IsSelectedForExitNode(netID) {
+	if m.routeSelector.IsSelected(netID) {
 		info.userSelected = append(info.userSelected, netID)
 	} else {
 		info.userDeselected = append(info.userDeselected, netID)

client/internal/routemanager/manager_v6exit_test.go

@@ -0,0 +1,47 @@
+package routemanager
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/internal/routeselector"
+	"github.com/netbirdio/netbird/route"
+)
+
+// TestUpdateRouteSelectorFromManagement_MirrorsV6ExitPair reproduces the bug seen
+// in netbird-engine.log: persisted selector state has the v4 exit node deselected
+// but its synthesized "-v6" pair explicitly selected (orphaned), so the ::/0 route
+// leaked onto the tunnel. The management update must mirror the v4 deselect onto the
+// v6 pair so FilterSelectedExitNodes drops it.
+func TestUpdateRouteSelectorFromManagement_MirrorsV6ExitPair(t *testing.T) {
+	const (
+		v4ID = route.NetID("Exit Node (raspberrypi)")
+		v6ID = route.NetID("Exit Node (raspberrypi)-v6")
+	)
+	all := []route.NetID{v4ID, v6ID}
+
+	rs := routeselector.NewRouteSelector()
+	// Orphan the v6 selection: select the pair, then deselect only the v4 base.
+	require.NoError(t, rs.SelectRoutes([]route.NetID{v4ID, v6ID}, true, all))
+	require.NoError(t, rs.DeselectRoutes([]route.NetID{v4ID}, all))
+	require.True(t, rs.IsSelected(v6ID), "precondition: orphaned v6 selection survives v4 deselect")
+
+	m := &DefaultManager{routeSelector: rs}
+
+	v4Route := &route.Route{NetID: v4ID, Network: netip.MustParsePrefix("0.0.0.0/0")}
+	v6Route := &route.Route{NetID: v6ID, Network: netip.MustParsePrefix("::/0")}
+	clientRoutes := route.HAMap{
+		"Exit Node (raspberrypi)|0.0.0.0/0": {v4Route},
+		"Exit Node (raspberrypi)-v6|::/0":   {v6Route},
+	}
+
+	m.updateRouteSelectorFromManagement(clientRoutes)
+
+	assert.False(t, rs.IsSelected(v6ID), "v6 pair must follow the v4 base deselect after the management update")
+
+	filtered := rs.FilterSelectedExitNodes(clientRoutes)
+	assert.Empty(t, filtered, "deselected v4 exit node must not leak its ::/0 pair onto the tunnel")
+}