NAT: Full Cone (EIM) NAT with ETCP_ID_NAT protocol, per-node routing, and comprehensive test suite

- New ETCP protocol ETCP_ID_NAT (0x02) for NAT traffic between nodes
- eim_nat.c/h: Pure NAT engine (table, port allocation, checksum updates, egress/ingress)
- nat_transport.c/h: Transport layer (NAT TUN, ETCP binding, client/provider roles)
- route_bgp_find_conn_for_node(): shared utility for optimal connection lookup
- tun_init_nat(): dedicated TUN creation for NAT
- Config: [nat] section with tun_ip, port_start/end, nat_via, forward rules
- Fix: csum_update_ip/transport use htonl() halves (correct on LE systems)
- Fix: IP_FRAG_MF_MASK changed to 0x2000 (after ntohs, MF is bit 13)
- Tests: 26 engine unit + 4 transport integration + 7 stress (29 total)

lib/debug_config.h

@@ -55,7 +55,8 @@ typedef int debug_category_t;
 #define DEBUG_CATEGORY_TRAFFIC     17  // Traffic flow (src/dst node IDs)
 #define DEBUG_CATEGORY_DEBUG       18  // Current debugging
 #define DEBUG_CATEGORY_GENERAL     19  // Messages for user (common log)
-#define DEBUG_CATEGORY_COUNT       20  // Total number of categories
+#define DEBUG_CATEGORY_NAT         20  // EIM NAT module
+#define DEBUG_CATEGORY_COUNT       21  // Total number of categories
 #define DEBUG_CATEGORY_ALL         (-1) // special value for all categories
 
 /* Debug configuration structure */