feat: add dual BLE+USB serial interface for Wio Tracker L1

BLE takes priority when connected; USB is always ready as fallback.
Both state machines run continuously — no BLE disconnect on USB connect.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/nrf52/DualSerialInterface.h

@@ -0,0 +1,49 @@
+#pragma once
+
+#include "../BaseSerialInterface.h"
+#include "../ArduinoSerialInterface.h"
+#include "SerialBLEInterface.h"
+
+// Wraps BLE + USB serial interfaces: BLE takes priority when connected,
+// USB is always ready as a fallback. Both state machines run continuously.
+class DualSerialInterface : public BaseSerialInterface {
+  SerialBLEInterface _ble;
+  ArduinoSerialInterface _usb;
+  uint8_t _ble_buf[MAX_FRAME_SIZE];
+  uint8_t _usb_buf[MAX_FRAME_SIZE];
+
+public:
+  void begin(const char* ble_prefix, char* node_name, uint32_t pin_code, Stream& usb_stream) {
+    _ble.begin(ble_prefix, node_name, pin_code);
+    _usb.begin(usb_stream);
+  }
+
+  void enable() override  { _ble.enable();  _usb.enable(); }
+  void disable() override { _ble.disable(); _usb.disable(); }
+  bool isEnabled() const override { return _ble.isEnabled() || _usb.isEnabled(); }
+
+  // Reports BLE connection state (used for UI indicator and buzzer Auto mode)
+  bool isConnected() const override { return _ble.isConnected(); }
+
+  bool isWriteBusy() const override {
+    return _ble.isConnected() ? _ble.isWriteBusy() : _usb.isWriteBusy();
+  }
+
+  size_t writeFrame(const uint8_t src[], size_t len) override {
+    return _ble.isConnected() ? _ble.writeFrame(src, len) : _usb.writeFrame(src, len);
+  }
+
+  size_t checkRecvFrame(uint8_t dest[]) override {
+    // Always pump both state machines: BLE needs this for TX queue drain and
+    // advertising watchdog; USB needs it to drain its input buffer.
+    size_t ble_len = _ble.checkRecvFrame(_ble_buf);
+    size_t usb_len = _usb.checkRecvFrame(_usb_buf);
+
+    if (_ble.isConnected()) {
+      if (ble_len > 0) { memcpy(dest, _ble_buf, ble_len); return ble_len; }
+    } else {
+      if (usb_len > 0) { memcpy(dest, _usb_buf, usb_len); return usb_len; }
+    }
+    return 0;
+  }
+};