Function write_line 

pub fn write_line(content: &str) -> Result<()>

Writes a line to stdout with pure LF (never CRLF), then flushes.

Uses write_all + explicit flush (rules: never rely on Drop alone). BrokenPipe is propagated so callers / main can exit 141.

Errors

Returns an error if stdout I/O fails (including BrokenPipe).