Legacy .ai export: radial gradients are written with their highlight (Bh)

The version 7 .ai writer wrote a radial gradient's highlight (Bh) only
when the gradient had a focal point off its center. A centered radial
gradient, such as an SVG radialGradient without fx and fy, was written
Bb, Bg, f, BB with no Bh. Every radial instance in the Adobe Illustrator File
Format Specification version 7.0, which the writer follows, has one,
0 0 0 0 Bh for a centered highlight (sections 3.5.5, 3.5.6 and 9.3).
#2687 reports that the reference app draws such a circle in its outer
color.

The writer now writes Bh in every radial instance, 0 0 0 0 Bh at the
center. A focal point past the radius, which an elliptical gradient
written circular can have, is written on the circle, so its x and y
offsets agree with the fraction 1 written after them (section 3.5.5).
Linear instances are unchanged. The result was not checked in the
reference app.

crates/engine/src/tests_legacy_ai.rs

@@ -192,6 +192,58 @@ fn art_under_transparent_art_stays_in_its_own_layer() {
     assert!(d.layers.iter().all(|l| l.children().is_some_and(|c| c.len() == 1)), "{:?}", d.layers);
 }
 
+/// #2687's file: a circle filled with a radial gradient from white to #1b2a6b, and a square filled
+/// with a linear one.
+const RADIAL_AND_LINEAR: &str = r##"<svg xmlns="http://www.w3.org/2000/svg" width="400" height="200" viewBox="0 0 400 200">
+  <defs>
+    <radialGradient id="r" cx="0.5" cy="0.5" r="0.5"><stop offset="0" stop-color="#ffffff"/><stop offset="1" stop-color="#1b2a6b"/></radialGradient>
+    <linearGradient id="l" x1="0" y1="0" x2="1" y2="0"><stop offset="0" stop-color="#ffffff"/><stop offset="1" stop-color="#1b2a6b"/></linearGradient>
+  </defs>
+  <circle cx="100" cy="100" r="80" fill="url(#r)"/>
+  <rect x="220" y="20" width="160" height="160" fill="url(#l)"/>
+</svg>"##;
+
+/// A radial gradient is written with its highlight, `0 0 0 0 Bh` at the center (§3.5.5, §3.5.6).
+/// #2687's file had none, and the reference app drew the circle in its outer color. The linear one
+/// has none. Both open again with their type, stops and place.
+#[test]
+fn a_radial_gradient_is_written_with_its_highlight() {
+    let mut s = Session::new();
+    let svg = vectorcraft_format::base64_encode(RADIAL_AND_LINEAR.as_bytes());
+    s.execute("document.open", &json!({"name": "t_radial.svg", "dataBase64": svg})).unwrap();
+    let r = s.execute("file.saveCopy", &json!({"format": "ai", "aiVersion": "7"})).unwrap();
+    let text = String::from_utf8(vectorcraft_format::base64_decode(r["dataBase64"].as_str().unwrap()).unwrap()).unwrap();
+    assert!(text.contains("\n%AI5_BeginGradient: (Gradient)\n(Gradient) 1 2 Bd\n[\n"), "{text}");
+    assert!(text.contains("\nBb\n0 0 0 0 Bh\n1 (Gradient) 100 100 0 80 1 0 0 1 0 0 Bg\nf\n0 BB\n"), "{text}");
+    assert!(text.contains("\nBb\n1 (Gradient 2) 220 180 0 160 1 0 0 1 0 0 Bg\nf\n0 BB\n"), "{text}");
+    assert_eq!(text.matches(" Bh\n").count(), 1, "{text}");
+    s.execute("document.open", &json!({"name": "t.ai", "dataBase64": r["dataBase64"]})).unwrap();
+    let d = &s.doc().unwrap().doc;
+    let mut found = vec![];
+    d.walk(|n| {
+        if let vectorcraft_color::Paint::Gradient(g) = n.appearance.fill_paint() {
+            found.push(*g);
+        }
+    });
+    let [radial, linear] = found.as_slice() else { panic!("{found:#?}") };
+    let navy = vectorcraft_color::Color::rgb(27.0 / 255.0, 42.0 / 255.0, 107.0 / 255.0);
+    use vectorcraft_color::GradientKind::{Linear, Radial};
+    for (g, kind, start, end) in [(radial, Radial, (100.0, 100.0), (180.0, 100.0)), (linear, Linear, (220.0, 20.0), (380.0, 20.0))] {
+        assert_eq!(g.gradient.kind, kind);
+        let [a, b] = g.gradient.stops.as_slice() else { panic!("{g:?}") };
+        assert_eq!((a.offset, b.offset, a.color), (0.0, 1.0, vectorcraft_color::Color::rgb(1.0, 1.0, 1.0)), "{g:?}");
+        assert!(b.color.to_rgb().iter().zip(navy.to_rgb()).all(|(x, y)| (x - y).abs() < 1e-3), "{g:?}");
+        let geom = g.geom.unwrap();
+        assert!((geom.start.x - start.0).abs() < 1e-3 && (geom.start.y - start.1).abs() < 1e-3, "{geom:?}");
+        assert!((geom.end.x - end.0).abs() < 1e-3 && (geom.end.y - end.1).abs() < 1e-3 && geom.focal.is_none(), "{geom:?}");
+    }
+    // White at the center, #1b2a6b at the edge.
+    for ((x, y), want) in [((100.0, 100.0), [255, 255, 255]), ((100.0, 21.0), [27, 42, 107])] {
+        let got = colour_at(d, x, y);
+        assert!(got.iter().zip(want).all(|(a, b)| a.abs_diff(b) <= 8), "{got:?} at {x}, {y}");
+    }
+}
+
 /// A `w`×`h` PNG of one colour (`rgba`), 72 ppi.
 fn png(w: u32, h: u32, rgba: [u8; 4]) -> Vec<u8> {
     let mut out = vec![];