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/ui-lvgl/QuickPanel.h

@@ -314,7 +314,7 @@ bool UITask::quickPanelOpen() const { return qpanel::s_open; }
 void UITask::quickPanelRefresh() {
   using namespace qpanel;
   if (!s_panel) return;
-  tileSet(T_BT, LV_SYMBOL_BLUETOOTH, hasConnection() ? "Connected" : "Bluetooth", isSerialEnabled());
+  tileSet(T_BT, LV_SYMBOL_BLUETOOTH, hasConnection() ? "Connected" : "Bluetooth", isBluetoothEnabled());
   tileSet(T_WIFI, LV_SYMBOL_WIFI, lvport::netRadio() == lvport::NET_UP ? "Online" : "WiFi", lvport::wifiAllowed());
   int32_t lat, lon;
   const bool gps = _core->gpsEnabled();
@@ -357,7 +357,7 @@ void UITask::quickPanelTile(int t, bool hold) {
   }
   // The tile shows what changed: no toast saying it again.
   switch (t) {
-    case T_BT:    setBluetooth(!isSerialEnabled()); break;
+    case T_BT:    setBluetooth(!isBluetoothEnabled()); break;
     case T_WIFI:  wifiSetAllowed(!lvport::wifiAllowed()); break;
     case T_GPS:   if (_core->gpsAvailable()) setGps(!_core->gpsEnabled()); break;
     case T_SOUND: