[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/routeselector/routeselector.go

@@ -7,6 +7,7 @@ import (
 	"sync"
 
 	"github.com/hashicorp/go-multierror"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/netbirdio/netbird/client/errors"
 	"github.com/netbirdio/netbird/route"
@@ -133,11 +134,14 @@ func (rs *RouteSelector) SyncPairedSelection(baseID, pairedID route.NetID) {
 	defer rs.mu.Unlock()
 
 	if rs.deselectAll {
+		log.Debugf("DIAG SyncPairedSelection: deselectAll set, skip base=%q paired=%q", baseID, pairedID)
 		return
 	}
 
 	_, baseSelected := rs.selectedRoutes[baseID]
 	_, baseDeselected := rs.deselectedRoutes[baseID]
+	_, pairedSelectedBefore := rs.selectedRoutes[pairedID]
+	_, pairedDeselectedBefore := rs.deselectedRoutes[pairedID]
 
 	delete(rs.selectedRoutes, pairedID)
 	delete(rs.deselectedRoutes, pairedID)
@@ -148,6 +152,18 @@ func (rs *RouteSelector) SyncPairedSelection(baseID, pairedID route.NetID) {
 	case baseDeselected:
 		rs.deselectedRoutes[pairedID] = struct{}{}
 	}
+
+	log.Debugf("DIAG SyncPairedSelection: base=%q (selected=%v deselected=%v) paired=%q before(selected=%v deselected=%v) -> after(selected=%v deselected=%v)",
+		baseID, baseSelected, baseDeselected, pairedID,
+		pairedSelectedBefore, pairedDeselectedBefore,
+		baseSelected, baseDeselected)
+}
+
+// MarshalSummary returns a short human-readable description of the selector state for diagnostics.
+func (rs *RouteSelector) MarshalSummary() string {
+	rs.mu.RLock()
+	defer rs.mu.RUnlock()
+	return fmt.Sprintf("deselectAll=%v selected=%v deselected=%v", rs.deselectAll, keysOf(rs.selectedRoutes), keysOf(rs.deselectedRoutes))
 }
 
 // FilterSelected removes unselected routes from the provided map.
@@ -186,24 +202,47 @@ func (rs *RouteSelector) FilterSelectedExitNodes(routes route.HAMap) route.HAMap
 		return route.HAMap{}
 	}
 
+	log.Debugf("DIAG FilterSelectedExitNodes: incoming %d networks, deselected=%v selected=%v deselectAll=%v",
+		len(routes), keysOf(rs.deselectedRoutes), keysOf(rs.selectedRoutes), rs.deselectAll)
+
 	filtered := make(route.HAMap, len(routes))
 	for id, rt := range routes {
 		netID := id.NetID()
 		if rs.isDeselectedLocked(netID) {
+			log.Debugf("DIAG FilterSelectedExitNodes: SKIP id=%q netID=%q (literally deselected)", id, netID)
 			continue
 		}
 
 		if !isExitNode(rt) {
+			log.Debugf("DIAG FilterSelectedExitNodes: KEEP id=%q netID=%q (not an exit node)", id, netID)
 			filtered[id] = rt
 			continue
 		}
 
+		log.Debugf("DIAG FilterSelectedExitNodes: EXITNODE id=%q netID=%q -> applyExitNodeFilter", id, netID)
 		rs.applyExitNodeFilter(id, netID, rt, filtered)
 	}
 
+	log.Debugf("DIAG FilterSelectedExitNodes: result keeps %d networks: %v", len(filtered), haKeysOf(filtered))
 	return filtered
 }
 
+func keysOf(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 haKeysOf(m route.HAMap) []route.HAUniqueID {
+	out := make([]route.HAUniqueID, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	return out
+}
+
 // MarshalJSON implements the json.Marshaler interface
 func (rs *RouteSelector) MarshalJSON() ([]byte, error) {
 	rs.mu.RLock()
@@ -286,15 +325,22 @@ func (rs *RouteSelector) applyExitNodeFilter(
 	rt []*route.Route,
 	out route.HAMap,
 ) {
+	log.Debugf("DIAG applyExitNodeFilter: id=%q netID=%q hasUserSel=%v isSelected=%v",
+		id, netID, rs.hasUserSelectionForRouteLocked(netID), rs.isSelectedLocked(netID))
 	if rs.hasUserSelectionForRouteLocked(netID) {
 		if rs.isSelectedLocked(netID) {
+			log.Debugf("DIAG applyExitNodeFilter: KEEP id=%q (netID %q is selected)", id, netID)
 			out[id] = rt
+		} else {
+			log.Debugf("DIAG applyExitNodeFilter: DROP id=%q (netID %q is deselected)", id, netID)
 		}
 		return
 	}
 
 	// no explicit selection for this route: defer to management's SkipAutoApply flag
 	sel := collectSelected(rt)
+	log.Debugf("DIAG applyExitNodeFilter: no user selection for netID %q; SkipAutoApply filter kept %d/%d routes for id=%q",
+		netID, len(sel), len(rt), id)
 	if len(sel) > 0 {
 		out[id] = sel
 	}