M3.98: radial annotator extent ellipse, aspect handle and off-centre focal point

Radial gradients show a dashed extent ellipse: its handle across the bar
sets the aspect ratio, the ellipse elsewhere rotates it, and the dot inside
the centre ring drags the focal point (GradientGeom.focal) for an off-centre
radial. paint.setGradientGeom takes aspect (%) and focal ([x,y] | null), and
start/end may be left out; the focal point keeps its place in the ellipse as
the vector, aspect and object transform. It renders as a two-point radial,
exports and imports as SVG fx/fy and PDF two-point radial shadings, expands
to off-centre rings, and travels in gradient params and the native format.

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

crates/engine/src/tests_gradient.rs

@@ -41,7 +41,7 @@ fn three_stops() -> GradientPaint {
             stop(1.0, Color::gray(0.25), 0.75, 0.5),
         ],
     });
-    g.geom = Some(GradientGeom { start: Point::new(10.0, 20.0), end: Point::new(110.0, 70.0), aspect: 0.5 });
+    g.geom = Some(GradientGeom { start: Point::new(10.0, 20.0), end: Point::new(110.0, 70.0), aspect: 0.5, focal: None });
     g.angle = g.geom.unwrap().angle_deg();
     g
 }