refactor: separate NAT detection from topology into standalone nat_detection module

- Extract NAT detection logic (~250 lines) from topo_group.c into new nat_detection.c
- NAT_DETECTION struct owns ping_pending, next_ping_req_id, allow_nat_check_local (removed from TOPO_GROUP)
- route_ping.* signatures changed: TOPO_GROUP* -> NAT_DETECTION* (+ TOPO_GROUP* where needed)
- topo_group.c now only dispatches BGP subcommands to nat_detection_* handlers
- UTUN_INSTANCE gets struct NAT_DETECTION* nat_det field (init/destroy in topo_groups_init/destroy)
- External callers (etcp_connections, etcp, control_server) call nat_detection_* directly
- nat_detection_cancel_for_conn / nat_detection_send_nat_info / nat_detection_link_ready / nat_detection_request_check_all
- route_ping_destroy_pending properly implemented (was dead code)
- TOPO_SUBCMD_PING_REQ/RESP moved from route_ping.h to topo_group.h (all BGP subcommands in one place)
- Tests updated: test_nat_detection, test_route_ping - both PASSED
- Build: auto(CORE_SOURCES+libutun) and cmake(chatgui/libutun)

tools/chatgui/src/memberlistmodel.cpp

@@ -45,6 +45,11 @@ void MemberListModel::setChannel(const QString& channelId) {
 
 void MemberListModel::refresh() { if (!m_channelId.isEmpty()) setChannel(m_channelId); }
 
+quint64 MemberListModel::nodeIdAt(int row) const {
+    auto it = m_cache.find(row);
+    return it != m_cache.end() ? it->nodeId : 0;
+}
+
 void MemberListModel::scheduleBatch(int centerRow) const {
     if (m_pendingFrom >= 0) return;
     int from = centerRow - BATCH / 2;