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/db/db_manager.cpp

@@ -273,9 +273,9 @@ QByteArray DbManager::getNodeEdPub(quint64 nodeId) const {
 QList<NodeAddr> DbManager::getOnlinePeerAddresses(quint64 excludeNodeId) const {
     QList<NodeAddr> list;
     sqlite3_stmt* stmt = prepareOrNull(
-        "SELECT na.node_id, na.family, na.protocol, na.address, na.port, na.rtt, na.is_nat"
+        "SELECT na.node_id, na.family, na.protocol, na.address, na.port, na.rtt, na.addr_type"
         " FROM node_addresses na JOIN nodes n ON na.node_id = n.node_id"
-        " WHERE na.is_nat=0 AND na.node_id != ?"
+        " WHERE na.addr_type=0 AND na.node_id != ?"
         " ORDER BY RANDOM()");
     if (!stmt) return list;
     sqlite3_bind_int64(stmt, 1, (sqlite3_int64)excludeNodeId);
@@ -287,7 +287,7 @@ QList<NodeAddr> DbManager::getOnlinePeerAddresses(quint64 excludeNodeId) const {
         a.address  = colBlob(stmt, 3);
         a.port     = (quint16)sqlite3_column_int(stmt, 4);
         a.rtt      = sqlite3_column_int(stmt, 5);
-        a.isNat    = sqlite3_column_int(stmt, 6) != 0;
+        a.addrType  = sqlite3_column_int(stmt, 6);
         list.append(a);
     }
     sqlite3_finalize(stmt);