Add exact IP:port match validation to NAT test

Test now verifies that NAT IP:port exactly matches client's local bind:
- Gets expected port from client socket using getsockname()
- Compares nat_port with expected_client_port for exact match
- Validates nat_ip equals 127.0.0.1 (localhost test)
- Fails test if values don't match exactly

This ensures the server correctly returns the client's actual address
in the INIT_RESPONSE handshake without any modification.

Test output:
  [TEST] Client socket bound to port 9012 (expected NAT port)
  [CLIENT] PASS: NAT address is set: 127.0.0.1:9012
  [CLIENT] PASS: nat_changes_count=0, nat_hits_count=0
  [CLIENT] PASS: NAT IP and port match exactly (127.0.0.1:9012)

tests/test_etcp_two_instances.c

@@ -5,6 +5,7 @@
 #include <time.h>
 #include <sys/stat.h>
 #include <arpa/inet.h>
+#include <sys/socket.h>
 
 #include "../src/etcp.h"
 #include "../src/etcp_connections.h"
@@ -24,6 +25,7 @@ static struct UASYNC* ua = NULL;
 static int test_completed = 0;  // 0 = running, 1 = success, 2 = timeout/failure
 static void* monitor_timeout_id = NULL;
 static void* test_timeout_id = NULL;
+static uint16_t expected_client_port = 0;  // Expected NAT port (client's local port)
 
 // Temp config file paths
 static char temp_dir[] = "/tmp/utun_test_XXXXXX";
@@ -183,16 +185,15 @@ static void monitor_connections(void* arg) {
                             return;
                         }
                         
-                        // Verify NAT port is in valid ephemeral port range (> 1024, typically)
-                        // Ephemeral ports are usually 32768-60999, but can be any > 1024
-                        if (link->nat_port <= 1024 || link->nat_port > 65535) {
-                            DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "[CLIENT] FAIL: NAT port %u is out of valid range (expected > 1024 and <= 65535)", 
-                                       link->nat_port);
+                        // Verify NAT port matches client's local port exactly (no NAT in test)
+                        if (expected_client_port != 0 && link->nat_port != expected_client_port) {
+                            DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "[CLIENT] FAIL: NAT port %u does not match expected port %u", 
+                                       link->nat_port, expected_client_port);
                             test_completed = 2;  // Failure
                             return;
                         }
                         
-                        DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[CLIENT] PASS: NAT IP and port contain valid values (127.0.0.1:%u)", 
+                        DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[CLIENT] PASS: NAT IP and port match exactly (127.0.0.1:%u)", 
                                    link->nat_port);
                     }
                 }
@@ -304,6 +305,18 @@ int main() {
     }
     DEBUG_INFO(DEBUG_CATEGORY_ETCP, "Client instance ready (node_id=%llx)\n\n", (unsigned long long)client_instance->node_id);
     
+    // Get client's local port from the socket (for NAT port validation)
+    if (client_instance->etcp_sockets) {
+        struct sockaddr_storage local_addr;
+        socklen_t addr_len = sizeof(local_addr);
+        if (getsockname(client_instance->etcp_sockets->fd, (struct sockaddr*)&local_addr, &addr_len) == 0) {
+            if (local_addr.ss_family == AF_INET) {
+                expected_client_port = ntohs(((struct sockaddr_in*)&local_addr)->sin_port);
+                DEBUG_INFO(DEBUG_CATEGORY_ETCP, "[TEST] Client socket bound to port %u (expected NAT port)", expected_client_port);
+            }
+        }
+    }
+    
     // Start monitoring
     DEBUG_INFO(DEBUG_CATEGORY_ETCP, "Starting connection monitoring...\n");
     monitor_timeout_id = uasync_set_timeout(ua, 100, NULL, monitor_connections);