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>

tests/fixtures/plot/README.md

@@ -0,0 +1,29 @@
+Page-setup fixtures saved by a commercial CAD application (2016 release, DXF
+AC1027 and the matching DWG). Provided by Panos for the plotting parity track;
+the drawing holds one 10 m line (`$INSUNITS` = 6, metres) and three paper
+layouts, each with a viewport of that line and its dimension:
+
+| Layout | Device | Media (`4`) | `44`×`45` | `73` | `72` | `142`/`143` | `147` | `75` |
+|---|---|---|---|---|---|---|---|---|
+| `A4 1-100` | `DWG To PDF.pc3` | `ISO_A4_(297.00_x_210.00_MM)` | 297 × 210 | 0 | mm | 1 / 1 | 25.4 | 16 |
+| `A3 - plotter` | system printer `HPB602DE (HP OfficeJet Pro 7740 series)` | `A3` | 297.01 × 419.99 | 1 | mm | 1 / 1 | 25.4 | 16 |
+| `ARCH D - PLOTTER` | the application's second PDF plotter configuration (`… PDF (High Quality Print).pc3`) | `ARCH_D_(36.00_x_24.00_Inches)` | 914.4 × 609.6 | 0 | in | 1 / 25.4 | 0.03937 | 16 |
+
+What the file establishes (see `src/app/update/file.rs`, `plot_settings_from_dialog`):
+
+- A landscape sheet is written either as the driver's own landscape medium
+  with rotation 0 (the PDF driver lists both orientations) or as a portrait
+  medium with rotation 90 (system printers) — rotation is relative to the
+  medium's orientation, and the medium's exact driver dimensions are kept.
+- Margins (`40`–`43`, mm) belong to the medium's own edges: the PDF driver's
+  A4 has 5.79375 left/right and 17.79375 bottom/top. Layout limits (`10`/`20`)
+  put the sheet at minus the *displayed* margins: at rotation 90 the medium's
+  top margin (20.02) becomes the layout's left one, its left margin (5.00) the
+  layout's bottom one.
+- Plot flags `70` = 672 on every layout: draw viewports first, print
+  lineweights, plot with plot styles — the "use standard scale" bit stays clear
+  even with a standard code in `75`; the model tab (fit to paper) sets it.
+- `147` is `142/143` × 25.4 on millimetre page setups and `142/143` itself on
+  inch ones.
+- An inch page setup over a millimetre paper space stores "1:1" as
+  `1 in = 25.4 units` (`75` = 16 kept).