[client] backport enforce a single selected exit node (#6640)

* routemanager: enforce a single selected exit node

Backport of the exit-node exclusivity reconcile from the 0.75.0 line
(upstream commit 966fbec11) onto v0.74.0. Exit nodes are mutually
exclusive, but the RouteSelector stores routes with default-on semantics,
so every available exit node reported as selected at once.

Reconcile exit-node selection on each network map: keep at most one
selected -- the user's persisted pick, else whatever management marks for
auto-apply (SkipAutoApply=false), else none. Never auto-activate an exit
node the map does not request.

Carries over only the manager/routeselector logic and its test; the
desktop-only client/server changes and the BumpNetworksRevision UI-push
feature from the original commit are intentionally excluded.

* routeselector: make exit-node reconciliation atomic

enforceSingleExitNode took the RouteSelector lock three separate times
(IsDeselectAll, then DeselectRoutes, then SelectRoutes), so a concurrent
DeselectAllRoutes could interleave and be silently undone: SelectRoutes on
its deselectAll branch clears the flag and re-selects the preferred exit
node, overriding the user's "all off".

Move the whole reconciliation into a single locked RouteSelector method
(SetExclusiveExitNode) that checks deselectAll inside the critical section,
so a deselect-all either fully precedes the reconcile (left untouched) or
fully follows it (honoured). No interleaving is possible.

client/internal/routeselector/routeselector.go

@@ -115,7 +115,38 @@ func (rs *RouteSelector) DeselectAllRoutes() {
 	clear(rs.selectedRoutes)
 }
 
-// IsDeselectAll reports whether the user has explicitly deselected all routes.
+// SetExclusiveExitNode atomically makes preferred the only selected exit node
+// among exitIDs: every other ID in exitIDs is deselected and preferred (when
+// non-empty) is selected, all under a single lock. Holding the lock across the
+// whole reconciliation prevents a concurrent DeselectAllRoutes from interleaving
+// between the deselect and select steps and being silently undone. A global
+// deselect-all is left untouched so the user's "all off" stays in effect;
+// non-exit routes are never referenced, so their selection is preserved.
+func (rs *RouteSelector) SetExclusiveExitNode(preferred route.NetID, exitIDs []route.NetID) {
+	rs.mu.Lock()
+	defer rs.mu.Unlock()
+
+	if rs.deselectAll {
+		return
+	}
+
+	for _, id := range exitIDs {
+		if id == preferred {
+			continue
+		}
+		rs.deselectedRoutes[id] = struct{}{}
+		delete(rs.selectedRoutes, id)
+	}
+
+	if preferred != "" {
+		delete(rs.deselectedRoutes, preferred)
+		rs.selectedRoutes[preferred] = struct{}{}
+	}
+}
+
+// IsDeselectAll reports whether the global "deselect all" flag is set, i.e. the
+// user explicitly disabled every route. Callers enforcing per-route invariants
+// (e.g. single exit node) should leave the selection untouched when it is.
 func (rs *RouteSelector) IsDeselectAll() bool {
 	rs.mu.RLock()
 	defer rs.mu.RUnlock()