fix(plot): write page setups in the file format's canonical form

One writer (plot_settings_from_dialog) now serves both the layout's embedded
settings and named page setups, so the two can no longer drift apart:

- media are stored as the portrait sheet with the orientation in the
  rotation code (0/90/180/270; upside-down adds a half turn) instead of
  swapped width/height with rotation limited to 0/180
- standard scales are written by their code (1:1 … 100:1) with matching
  numerator/denominator; other scales as a readable custom ratio in the
  layout's own paper unit, converting from the drawing's scale-family unit
  (a metre drawing plotting "1:100" stores 10 mm = 1 unit, an inch layout
  stores 1 in = 2540 units); reading converts back and keeps a ratio the
  scale list has no name for instead of snapping to 1:1
- the layout's paper unit and every flag the dialog does not edit survive a
  round trip untouched; layout limits follow the paper-space unit
- new drawings, new layouts, CLEAR and the automation "new" op share one
  default page setup (canonical A4 name, rotation 90, none_device, 1:1
  layout plot) through Scene::reset_to_new_drawing

Scene gains scale_family_unit_mm, scale_ratio and a scale-name ratio parser
(ratios and architectural names). Round-trip tests cover orientation,
standard/custom/fit scales, unit conversion, named-setup parity, preserved
flags and the default layout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scene/page_setup.rs

@@ -5,9 +5,41 @@
 //! dictionary + objects consistent so the result round-trips through DWG/DXF.
 
 use super::Scene;
-use acadrust::objects::{Dictionary, ObjectType, PlotSettings};
+use acadrust::objects::{Dictionary, Layout, ObjectType, PlotSettings};
 use acadrust::Handle;
 
+/// Give a freshly created paper layout the page setup a new drawing starts
+/// with — ISO A4 landscape on no plotter, plotted 1:1 as a layout — stored the
+/// way the file format expects it: the portrait medium under its canonical
+/// name with the orientation in the rotation code. `plot_style` names the
+/// plot style table to attach, or is empty.
+pub fn apply_default_page_setup(layout: &mut Layout, plot_style: &str) {
+    use acadrust::objects::{PlotPaperUnits, PlotRotation, PlotType, ScaledType};
+    let a4 = crate::io::paper_catalog::default_paper();
+    let (paper_width, paper_height) = a4.portrait_mm();
+    layout.min_limits = (0.0, 0.0);
+    layout.max_limits = (paper_height, paper_width);
+    layout.min_extents = (0.0, 0.0, 0.0);
+    layout.max_extents = (paper_height, paper_width, 0.0);
+    layout.paper_width = paper_width;
+    layout.paper_height = paper_height;
+    layout.plot_rotation = PlotRotation::Degrees90.to_code();
+    layout.plot_paper_units = PlotPaperUnits::Millimeters.to_code();
+    layout.plot_type = PlotType::Layout.to_code();
+    layout.plot_scale_type = ScaledType::OneToOne.to_code();
+    layout.plot_scale_numerator = 1.0;
+    layout.plot_scale_denominator = 1.0;
+    layout.plot_scale_factor = 1.0;
+    layout.plot_flags.use_standard_scale = true;
+    layout.plot_flags.print_lineweights = true;
+    layout.plot_flags.draw_viewports_first = true;
+    layout.plot_flags.plot_plot_styles = !plot_style.is_empty();
+    layout.plot_flags.show_plot_styles = !plot_style.is_empty();
+    layout.plot_style_sheet = plot_style.to_string();
+    layout.paper_size = a4.canonical.to_string();
+    layout.plot_printer_name = crate::io::plot_device::PlotDevice::None.canonical_name();
+}
+
 impl Scene {
     /// Handle of the `ACAD_PLOTSETTINGS` dictionary, located robustly.
     ///