feat: plot a model-space window to PDF

Pick a rectangular window in model space with the existing PLOTWINDOW command (now enabled in model space, previously paper-space only) and export it to a PDF at a chosen ISO paper size (A4-A0), orientation and scale (fit or ratio). The window is a hard clip boundary. Adds paper-size presets, a window-to-sheet transform, scale + clip support in the PDF exporter, and Page Setup controls (format/orientation/pick window).

src/io/mod.rs

@@ -14,6 +14,7 @@ pub mod xref;
 pub mod linetypes;
 pub mod patterns;
 pub mod update_check;
+pub mod paper_sizes;
 
 use crate::scene::DerivedCaches;
 use acadrust::entities::EntityType;