refactor(companion): the app's links through upstream's MultiSerialInterface

- Bluetooth + USB (+ WiFi/Ethernet/UART per build) in one interface_manager,
  as upstream; frames go to every enabled link, USB is read while BT is
  connected. DUAL_SERIAL now means ENABLE_USB_INTERFACE.
- WiFi interface with upstream's wifi_* prefs and set/get wifi.* CLI.
- isClientConnected moved into ArduinoSerialInterface (DTR on nRF52 and
  ESP32 native CDC).
- UI uses isBluetoothEnabled/enableBluetooth/disableBluetooth; turning BT
  off leaves USB and the other links on.
- DualSerialInterface removed.
- Sim build gains DynamicConfigSerializer.cpp (broken since bd64c203).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/MyMesh.cpp

@@ -1372,6 +1372,9 @@ MyMesh::MyMesh(mesh::Radio &radio, mesh::RNG &rng, mesh::RTCClock &rtc, SimpleMe
   _prefs.clearFields();
   memset(_prefs.ch_scope_idx, NodePrefs::CH_SCOPE_DEFAULT, sizeof(_prefs.ch_scope_idx));
   _prefs.airtime_factor = 1.0;
+#ifdef ENABLE_WIFI_INTERFACE
+  _prefs.wifi_enabled = 1;   // on, so a WiFi-only build works; it doesn't start without an SSID
+#endif
   _prefs.tx_delay_factor = 0.5f;
   _prefs.direct_tx_delay_factor = 0.2f;
   strcpy(_prefs.node_name, "NONAME");
@@ -1570,7 +1573,7 @@ bool MyMesh::isValidClientRepeatFreq(uint32_t f) const {
 
 void MyMesh::startInterface(BaseSerialInterface &serial) {
   _serial = &serial;
-  if (!_prefs.ble_off) serial.enable();
+  serial.enable();   // main.cpp then turns Bluetooth off if the user left it off (ble_off)
 }
 
 static bool isAllZero(const uint8_t* buf, size_t n) {