Function write_stdout_line 

pub fn write_stdout_line(line: &str) -> Result<()>

Write a line to stdout (Control Plane).

Writes the string followed by a single \n and flushes.

Important

• Uses explicit \n, NOT println! (which may add \r\n on Windows)
• Flushes immediately (parent waits for complete line)
• Any logging should go to stderr, not stdout

Errors

Returns IO error if write or flush fails.