Fix missing STX (0x02) DLE-escape in transmit()

[brocci]

Summary

• Fix missing DLE escape for STX (0x02) in transmit() per NMRA CMRInet spec LCS-9.10.1 v1.1
• The spec requires DLE (0x10) to be inserted before any data byte equal to STX (0x02), ETX (0x03), or DLE (0x10) in the message body
• The current code only escapes ETX and DLE — an output data byte of 0x02 is sent raw, and the host (JMRI) may misinterpret it as start-of-text, breaking packet synchronization
• Also fixes a misleading comment

Details

Problem: The NMRA CMRInet specification (LCS-9.10.1, §Data Link Escape (DLE) Processing) requires:

"Software must insert a DLE in front of any of these data values when forming a Transmit Data or Receive Data message."

The three values requiring DLE are STX (0x02), ETX (0x03), and DLE (0x10).

The current transmit() only checks two of the three:

if (_tx_buffer[i] == ETX)
    _serial.write(ESC);
if (_tx_buffer[i] == ESC)
    _serial.write(ESC);
_serial.write(_tx_buffer[i]);

STX (0x02) is not escaped. If any output data byte carries the value 0x02 (perfectly valid binary data), it is transmitted raw. A CMRInet host receiving this frame will see 0x02 in the data stream and interpret it as the start-of-text character, potentially corrupting the current frame and subsequent ones.

Fix: Combine the escape check into a single condition covering all three protocol character values (STX, ETX, DLE). The ESC constant in the code is already defined as 0x10 (DLE).

Changes

CMRI.cpp — transmit() method:

 	for (int i=0; i<_tx_length; i++)
 	{
		if (_tx_buffer[i] == STX || _tx_buffer[i] == ETX || _tx_buffer[i] == ESC)
			_serial.write(ESC);
 		_serial.write(_tx_buffer[i]);
 	}

Closes #18