test(plot): pin the paper-scale tests to millimetre drawing units

New drawings now default to metre insertion units, and the plot dialog
reads a ratio against the drawing's units, so these tests (written for
millimetres) saw 1:100 as 10 mm per unit. The metre case keeps its own
test.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/file.rs

@@ -5645,6 +5645,9 @@ mod plot_paper_tests {
         app.automation_op(r#"{"op":"new"}"#);
         let _ = app.update(Message::LayoutSwitch("Layout1".into()));
         let i = app.active_tab;
+        // Millimetre drawing units, so a ratio plots as written; the metre
+        // case has its own test.
+        app.tabs[i].scene.document.header.insertion_units = 4;
         let mut ps = app.tabs[i]
             .scene
             .plot_settings_for("Layout1")