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-tiny/UITask.cpp

@@ -241,7 +241,7 @@ public:
     } else if (_page == HomePage::BLUETOOTH) {
       display.setColor(DisplayDriver::GREEN);
       display.drawXbm((display.width() - 32) / 2, 8,
-          _task->isSerialEnabled() ? bluetooth_on : bluetooth_off,
+          _task->isBluetoothEnabled() ? bluetooth_on : bluetooth_off,
           32, 32);
       display.setTextSize(1);
       // display.drawTextCentered(display.width() / 2, 40 - 11, "toggle: " PRESS_LABEL);
@@ -387,10 +387,10 @@ public:
       return true;
     }
     if (c == KEY_ENTER && _page == HomePage::BLUETOOTH) {
-      if (_task->isSerialEnabled()) {  // toggle Bluetooth on/off
-        _task->disableSerial();
+      if (_task->isBluetoothEnabled()) {  // toggle Bluetooth on/off
+        _task->disableBluetooth();
       } else {
-        _task->enableSerial();
+        _task->enableBluetooth();
       }
       return true;
     }
@@ -674,7 +674,7 @@ void UITask::loop() {
         _cached_batt_mv,
         isBuzzerQuiet(),
         getGPSState(),
-        isSerialEnabled());
+        isBluetoothEnabled());
 
     bool status_dirty = _statusBar.needsRedraw();
     bool content_dirty = (millis() >= _next_refresh && curr);