Function render_errors_plain 

pub fn render_errors_plain(
    errors: &[CompileError],
    source: &str,
    filename: &str,
) -> String

Render a list of compile errors to a string with colour disabled and the given filename as the source label. Unlike render_errors, the output contains no ANSI escape codes, so it is byte-stable — suitable for the committed diagnostic transcripts under site/src/diagnostics/.