XModemManagerImpl
XModem-CRC receiver state machine.
Protocol summary (device = sender, Android = receiver):
SOH / STX → data block with seq, CRC-CCITT-16, payload; reply ACK or NAK
EOT → end of transfer; reply ACK, emit assembled file
CAN → sender cancelled; reset state
CRC algorithm: CRC-CCITT (poly 0x1021, init 0x0000), same as the Meshtastic firmware.