Keep rectangle opposite corners free of ortho (STRETCH window and friends)

With ORTHOMODE on, the opposite corner of STRETCH's crossing window was bent
onto an axis, so the window collapsed and caught nothing. The prompt set only
the rubber-band base; it now marks it as a window corner (Prompt::corner), as
ZOOM Window and RECTANG already do.

Same fix for the other two-corner rectangles: the Window/Crossing/Box selection
modes, PLOTWINDOW, MVIEW (and its Restore), -VIEW Window and REVCLOUD
Rectangular. Regression tests in crates/engine/tests/stretch_window_ortho.rs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/plotters.rs

@@ -365,7 +365,7 @@ impl Interactive for PlotWindowM {
     fn prompt(&self, _s: &Session) -> Prompt {
         match self.first {
             None => Prompt::new("Specify first corner", Accept::POINT),
-            Some(a) => Prompt::new("Specify opposite corner", Accept::POINT).base(a),
+            Some(a) => Prompt::new("Specify opposite corner", Accept::POINT).corner(a),
         }
     }
     fn input(&mut self, s: &mut Session, i: Input) -> Result<Step> {