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 <[email protected]>

src/scene/entity.rs

@@ -2709,4 +2709,25 @@ impl Scene {
         self.named_parameters = crate::scene::named_parameters::ParameterTable::new();
         self.bump_geometry();
     }
+
+    /// Reset to the drawing File → New produces. Every path that starts a
+    /// fresh drawing (the New tab, CLEAR, the automation `new` op) goes
+    /// through here so they all agree on what a blank drawing contains.
+    pub fn reset_to_new_drawing(&mut self) {
+        self.clear();
+        self.populate_new_drawing_defaults();
+    }
+
+    /// What a blank document gets on top of the codec's defaults: the
+    /// standard linetypes and a compatible page setup on every paper layout.
+    pub fn populate_new_drawing_defaults(&mut self) {
+        crate::io::linetypes::populate_document(&mut self.document);
+        for obj in self.document.objects.values_mut() {
+            if let acadrust::objects::ObjectType::Layout(l) = obj {
+                if l.name != "Model" {
+                    crate::scene::apply_default_page_setup(l, "");
+                }
+            }
+        }
+    }
 }