fix(plot): follow the fixture's page-setup conventions

A drawing saved by a commercial CAD application (tests/fixtures/plot,
provided by Panos: one 10 m line, three layouts) corrects several
assumptions of the previous commit:

- rotation is relative to the medium's own orientation: an unchanged sheet
  keeps the driver's exact medium (its PDF driver lists landscape media of
  its own, written with rotation 0; a system printer's portrait A3 plotted
  landscape carries 90); only a newly picked sheet is written as the
  catalogue's portrait medium
- the stored scale factor (DXF 147) is the ratio × 25.4 on millimetre page
  setups and the plain ratio on inch ones; the "use standard scale" bit
  stays clear for standard and custom scales and is set for fit-to-paper;
  a standard code is kept across a paper-unit conversion ("1:1" on an
  inch page setup over a millimetre paper space is 1 in = 25.4 units)
- layout plots take the same scale path as every other area, so a foreign
  layout's paper-space unit survives
- "plot with plot styles" / "display plot styles" round-trip as the
  dialog's checkboxes, independent of a style table being named
- layout limits place the sheet at minus its displayed margins, the way the
  source application does (the printable corner is the paper-space origin);
  the margin ↔ rotation mapping shared by the printable-area guide now
  turns the medium's top margin into the layout's left one at 90°

The fixture round-trips through the dialog with every stored field
unchanged; picking a new sheet on it writes the canonical form.

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

src/scene/mod.rs

@@ -33,7 +33,7 @@ pub(crate) mod centermark;
 pub(crate) mod dimension_assoc;
 pub(crate) mod dimension_assoc_chain;
 pub use dimension_assoc::{ReferenceStatus, ResolvedReference};
-pub use page_setup::apply_default_page_setup;
+pub use page_setup::{apply_default_page_setup, rotated_margins};
 mod dwg_native_constraints;
 mod entity;
 #[cfg(test)]
@@ -4362,13 +4362,8 @@ impl Scene {
             None
         })?;
         // `paper_limits()` already swaps the sheet for a 90°/270° rotation, so the
-        // margins must rotate to the same edges: a margin on a physical side moves
-        // to the displayed side that side rotates onto.
-        let (ml, mb, mr, mt) = match rot {
-            1 | 3 => (bottom, left, top, right),
-            2 => (right, top, left, bottom),
-            _ => (left, bottom, right, top),
-        };
+        // margins must rotate to the same edges.
+        let (ml, mb, mr, mt) = page_setup::rotated_margins((left, bottom, right, top), rot);
         // Plot margins are millimetres like the paper size; scale them into the
         // layout's paper-space units so the inset matches the (already scaled)
         // sheet rect. Without this an inch paper space insets an ~8-inch sheet

src/scene/page_setup.rs

@@ -40,6 +40,25 @@ pub fn apply_default_page_setup(layout: &mut Layout, plot_style: &str) {
     layout.plot_printer_name = crate::io::plot_device::PlotDevice::None.canonical_name();
 }
 
+/// The unprintable margins of a sheet as they appear on the rotated layout,
+/// `(left, bottom, right, top)`. Stored margins belong to the medium's own
+/// edges; a plot rotated by a quarter turn (`rotation` is the file's 0–3
+/// code) shows the medium turned on the layout, so each margin moves to the
+/// edge its side lands on — verified against limits written by the
+/// commercial application: at 90° the medium's top margin is the layout's
+/// left one and its left margin the layout's bottom one.
+pub fn rotated_margins(
+    (left, bottom, right, top): (f64, f64, f64, f64),
+    rotation: i16,
+) -> (f64, f64, f64, f64) {
+    match rotation {
+        1 => (top, left, bottom, right),
+        2 => (right, top, left, bottom),
+        3 => (bottom, right, top, left),
+        _ => (left, bottom, right, top),
+    }
+}
+
 impl Scene {
     /// Handle of the `ACAD_PLOTSETTINGS` dictionary, located robustly.
     ///