Function run_build 

pub(crate) fn run_build(entry: &Path, open: PdfOpen<'_>) -> ExitCode

mos build: read source, parse, lower, lay out, and emit a PDF to build/<entry-stem>.pdf. MVP 0 produces a fixed-A4 document using the standard PDF base fonts (no embedding). Layout warnings (e.g. non-ASCII substitutions) print but don’t fail the build.