bgp: add test_bgp_triangle + keepalive_adaptive config option

- keepalive_adaptive=0 disables adaptive keepalive period growth,
  keeping it fixed at 200ms for faster timeout detection in tests
- test_bgp_triangle: 5-node triangle topology test (6 phases):
  validates alternative path recording (fix#1), cascading node
  removal on full isolation, and full recovery
- config generation via utun_instance_create_from_str() with
  pre-generated keys — no temp files needed

src/config_parser.c

@@ -377,6 +377,10 @@ static int parse_global(const char *key, const char *value, struct global_config
         global->keepalive_interval = atoi(value);
         return 0;
     }
+    if (strcmp(key, "keepalive_adaptive") == 0) {
+        global->keepalive_adaptive = atoi(value);
+        return 0;
+    }
     if (strcmp(key, "bbr_max_cwnd") == 0) {
         global->bbr_max_cwnd = atoi(value);
         return 0;
@@ -411,7 +415,7 @@ static int parse_global(const char *key, const char *value, struct global_config
         global->tun_test_mode = atoi(value);
         return 0;
     }
-    DEBUG_ERROR(DEBUG_CATEGORY_CONFIG, "%s:%d: Unknown global option '%s'. Valid: my_node_name, my_private_key, my_public_key, my_node_id, tun_ifname, tun_ip, mtu, keepalive_timeout, keepalive_interval, bbr_max_cwnd, debug_level, log_file, db_path, enable_timestamp, enable_function_names, enable_file_lines, enable_colors, tun_test_mode", filename, line_num, key);
+    DEBUG_ERROR(DEBUG_CATEGORY_CONFIG, "%s:%d: Unknown global option '%s'. Valid: my_node_name, my_private_key, my_public_key, my_node_id, tun_ifname, tun_ip, mtu, keepalive_timeout, keepalive_interval, keepalive_adaptive, bbr_max_cwnd, debug_level, log_file, db_path, enable_timestamp, enable_function_names, enable_file_lines, enable_colors, tun_test_mode", filename, line_num, key);
     return -1;
 }
 
@@ -741,6 +745,7 @@ static struct utun_config* parse_config_internal(FILE *fp, const char *filename)
     cfg->global.name[0] = '\0';
     cfg->global.keepalive_timeout = 2000;  // Default 2 seconds
     cfg->global.keepalive_interval = 200;   // Default 0.2 s
+    cfg->global.keepalive_adaptive = 1;     // Default: adaptive enabled
     cfg->global.bbr_max_cwnd = 1048576; // Default 1MB (was INFLIGHT_LIM_MAX)
     cfg->global.tcp_recv_buf = 0;      // 0 = OS default
     cfg->global.firewall_rules = NULL;