- Add USB Host permissions and device filter to AndroidManifest.xml
- Create UsbSerialPlugin Kotlin plugin for USB Host API (enumerate, permission, open devices)
- Add serial_connect_fd command for Android to accept USB file descriptors
- Create RawFdStream wrapper for async I/O on raw file descriptors
- Make run_serial_with_poll generic over AsyncRead+AsyncWrite
- Register UsbSerialPlugin in MainActivity