Add independent STUN NAT probes with RFC 5780 discovery

src/utun_instance.h

@@ -69,6 +69,7 @@ struct CONN_MGR;
 struct ETCP_CONNECT;
 struct DB_SYNC;
 struct NAT_DETECTION;
+struct STUN_NAT;
 struct merkle_sync;
 
 struct NETWORK_ENTRY {
@@ -139,6 +140,7 @@ struct UTUN_INSTANCE {
     struct TOPO_GROUPS* topo_groups;    // Groups module for topology exchange
     sqlite3* topo_sqlite_db;            // Shared SQLite DB (nodes/channels/peers)
     struct NAT_DETECTION* nat_det;      // NAT detection module
+    struct STUN_NAT* stun_nat;          // независимая локальная STUN-диагностика UDP
     struct merkle_sync* msync;         // Merkle tree sync module (per-instance)
     
     // Identification