mapper: keep pings when a full update collapses pending changes

A full renders state at drain time but cannot carry a one-shot
PingRequest; queue each ping as a ping-only frame after the full.

hscontrol/mapper/node_conn.go

@@ -262,6 +262,14 @@ func (mc *multiChannelNodeConn) prependPending(changes ...change.Change) {
 	mc.pendingMu.Unlock()
 }
 
+// collapsePendingToFull replaces pending, together with incoming, by a
+// single full update and the pings it cannot carry ([change.CollapseToFull]).
+func (mc *multiChannelNodeConn) collapsePendingToFull(incoming []change.Change) {
+	mc.pendingMu.Lock()
+	mc.pending = change.CollapseToFull(mc.id, slices.Concat(mc.pending, incoming))
+	mc.pendingMu.Unlock()
+}
+
 // drainPending atomically removes and returns all pending changes.
 // Returns nil if there are no pending changes.
 func (mc *multiChannelNodeConn) drainPending() []change.Change {