XModemAdapter: ensure file truncation before receiving and add isBusy() method to prevent concurrent writes

src/xmodem.h

@@ -52,6 +52,12 @@ class XModemAdapter
     meshtastic_XModem getForPhone();
     void resetForPhone();
 
+    // True while a file is being received from or transmitted to the phone.
+    // Callers (e.g. NodeDB::saveNodeDatabaseToDisk) consult this to avoid
+    // opening the same on-disk file for write in parallel, which races our
+    // long-lived xmodem `file` handle and ends up producing a 0-byte file.
+    bool isBusy() const { return isReceiving || isTransmitting; }
+
   private:
     bool isReceiving = false;
     bool isTransmitting = false;