Separate debug messages from stderr using serial port file descriptor

[pbalduino]

Problem

Currently, debug messages are written to stderr using fprintf, which causes:

• Debug messages mixed with actual error messages on stderr
• Console clutter
• Debug messages not properly captured in log files

Proposed Solution

Add a file descriptor for the serial port and route debug messages through this descriptor instead of stderr:

• Create a dedicated file descriptor for the serial port
• Update fprintf calls for debug messages to use the serial port descriptor
• Keep STDERR clean for only actual error messages
• Enable debug messages to be visible in log files

Benefits

• Cleaner separation between debug logs and error messages
• Less console clutter
• Better log file management
• Easier debugging workflow