[client] add DIAG logging to trace v6 exit-pair mirror

The write-time mirror did not eliminate the leak in field testing. Re-add the
DIAG diagnostics around the exit-node selection flow to capture a fresh trace:

- UpdateRoutes: incoming client networks, selector state before/after the
  management update, and the networks remaining after FilterSelectedExitNodes.
- mirrorV6ExitPairSelections: the NetIDs present in this update and the v6 pairs
  V6ExitMergeSet derives from them (reveals whether the v4 base and its ::/0 pair
  are present in the same update so the pair can be matched).
- SyncPairedSelection: the base/paired state before and after the sync.
- FilterSelectedExitNodes / applyExitNodeFilter: per-route SKIP/KEEP/DROP and the
  selection lookups behind each decision.
- updateExitNodeSelections / logExitNodeUpdate: categorization and deselect set.

Temporary; to be removed once the root cause is confirmed.

client/internal/routemanager/manager.go

@@ -429,10 +429,16 @@ func (m *DefaultManager) UpdateRoutes(
 	var merr *multierror.Error
 	if !m.disableClientRoutes {
 
+		log.Debugf("DIAG UpdateRoutes: incoming %d client networks: %v", len(clientRoutes), haKeysList(clientRoutes))
+		log.Debugf("DIAG UpdateRoutes: selector BEFORE management update: %s", m.routeSelector.MarshalSummary())
+
 		// Update route selector based on management server's isSelected status
 		m.updateRouteSelectorFromManagement(clientRoutes)
 
+		log.Debugf("DIAG UpdateRoutes: selector AFTER management update: %s", m.routeSelector.MarshalSummary())
+
 		filteredClientRoutes := m.routeSelector.FilterSelectedExitNodes(clientRoutes)
+		log.Debugf("DIAG UpdateRoutes: AFTER filter, %d networks remain: %v", len(filteredClientRoutes), haKeysList(filteredClientRoutes))
 
 		if err := m.updateSystemRoutes(filteredClientRoutes); err != nil {
 			merr = multierror.Append(merr, fmt.Errorf("update system routes: %w", err))
@@ -724,12 +730,38 @@ func (m *DefaultManager) mirrorV6ExitPairSelections(clientRoutes route.HAMap) {
 		routesByNetID[haID.NetID()] = routes
 	}
 
-	for v6ID := range route.V6ExitMergeSet(routesByNetID) {
+	v6Pairs := route.V6ExitMergeSet(routesByNetID)
+	log.Debugf("DIAG mirrorV6ExitPairSelections: netIDs=%v v6Pairs=%v", netIDKeysList(routesByNetID), netIDSetList(v6Pairs))
+	for v6ID := range v6Pairs {
 		baseID := route.NetID(strings.TrimSuffix(string(v6ID), route.V6ExitSuffix))
 		m.routeSelector.SyncPairedSelection(baseID, v6ID)
 	}
 }
 
+func netIDKeysList(m map[route.NetID][]*route.Route) []route.NetID {
+	out := make([]route.NetID, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}
+
+func netIDSetList(m map[route.NetID]struct{}) []route.NetID {
+	out := make([]route.NetID, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}
+
+func haKeysList(m route.HAMap) []route.HAUniqueID {
+	out := make([]route.HAUniqueID, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}
+
 type exitNodeInfo struct {
 	allIDs               []route.NetID
 	selectedByManagement []route.NetID
@@ -784,6 +816,8 @@ func (m *DefaultManager) checkManagementSelection(routes []*route.Route, netID r
 
 func (m *DefaultManager) updateExitNodeSelections(info exitNodeInfo) {
 	routesToDeselect := m.getRoutesToDeselect(info.allIDs)
+	log.Debugf("DIAG updateExitNodeSelections: allIDs=%v userSelected=%v userDeselected=%v selectedByManagement=%v -> routesToDeselect(no user selection)=%v",
+		info.allIDs, info.userSelected, info.userDeselected, info.selectedByManagement, routesToDeselect)
 	m.deselectExitNodes(routesToDeselect)
 	m.selectExitNodesByManagement(info.selectedByManagement, info.allIDs)
 }
@@ -823,4 +857,6 @@ func (m *DefaultManager) selectExitNodesByManagement(selectedByManagement []rout
 func (m *DefaultManager) logExitNodeUpdate(info exitNodeInfo) {
 	log.Debugf("Updated route selector: %d exit nodes available, %d selected by management, %d user-selected, %d user-deselected",
 		len(info.allIDs), len(info.selectedByManagement), len(info.userSelected), len(info.userDeselected))
+	log.Debugf("DIAG logExitNodeUpdate: allIDs=%v selectedByManagement=%v userSelected=%v userDeselected=%v",
+		info.allIDs, info.selectedByManagement, info.userSelected, info.userDeselected)
 }