Tests: mixed spot gradients export as DeviceN shadings when radial or in a Lab ink

- A radial gradient from a spot swatch to a CMYK color is written as a
  DeviceN shading (ShadingType 3) of Yellow and the spot ink.
- A Lab spot color used as a flat fill and in a mixed gradient is
  written as one Separation space with the Lab alternate, which the
  DeviceN space names under /Colorants.
- docs/roadmap.md lists the gradients that still export without the
  spot ink: mixed spot/process gradients in SVG and PDF gradients with
  RGB or Lab stops. docs/target-app-parity.md also notes that a
  Perceptual gradient is written to PDF as an image.

crates/engine/src/tests/labspots.rs

@@ -202,3 +202,27 @@ fn lab_spots_export_with_a_lab_alternate() {
     assert!(text.contains("/Separation/Lab#20Ink/DeviceCMYK") && !text.contains("/Lab<<"));
     assert_xref_valid(&pdf);
 }
+
+#[test]
+fn lab_spots_in_device_n_gradients_keep_one_alternate() {
+    let _g = GLOBAL.lock().unwrap_or_else(|e| e.into_inner());
+    let (mut s, _) = lab_ink();
+    let g = NodeId(run(&mut s, "shape.rectangle", json!({"x": 50, "y": 10, "width": 30, "height": 30}))["id"].as_u64().unwrap());
+    run(
+        &mut s,
+        "paint.setFill",
+        json!({"ids": [g.0], "gradient": {"stops": [{"offset": 0, "swatch": "Lab Ink"}, {"offset": 1, "color": {"c": 0, "m": 0, "y": 0.6, "k": 0}}]}}),
+    );
+    let pdf = pdf_bytes(&s);
+    let text = String::from_utf8_lossy(&pdf);
+    // One Separation space for the ink, with the Lab alternate: the flat fill paints in it, and the
+    // gradient's DeviceN space names it as the ink's colorant.
+    assert_eq!(text.matches("[/Separation/Lab#20Ink").count(), 1, "one Separation space for the ink");
+    assert!(!text.contains("/Separation/Lab#20Ink/DeviceCMYK"), "no CMYK Separation space for the ink");
+    let at = text.find("[/Separation/Lab#20Ink[/Lab<<").expect("the ink's Lab Separation space");
+    let obj = text[..at].trim_end().strip_suffix("obj").and_then(|h| h.trim_end().rsplit('\n').next()).expect("the space's object header");
+    let id = obj.split_whitespace().next().expect("the space's object number");
+    assert!(text.contains("/DeviceN[/Yellow/Lab#20Ink]"), "a DeviceN shading of Yellow and the ink");
+    assert!(text.contains(&format!("/Colorants<</Lab#20Ink {id} 0 R>>")), "the DeviceN space's colorant is the ink's Lab Separation space");
+    assert_xref_valid(&pdf);
+}

crates/engine/src/tests/linked_stops.rs

@@ -166,6 +166,20 @@ fn spot_gradients_mixed_with_other_inks_export_as_device_n_shadings() {
     assert!(!warnings.iter().any(|w| w.contains("spot")), "{warnings:?}");
 }
 
+#[test]
+fn radial_spot_gradients_mixed_with_other_inks_export_as_device_n_shadings() {
+    let (mut s, _) = setup();
+    run(
+        &mut s,
+        "paint.editGradient",
+        json!({"kind": "radial", "stops": [{"offset": 0, "swatch": "Ink"}, {"offset": 1, "color": {"c": 0, "m": 0, "y": 0.6, "k": 0}}]}),
+    );
+    let (pdf, warnings) = pdf_of(&s);
+    assert!(pdf.contains("/ShadingType 3") && pdf.contains("/DeviceN[/Yellow/Ink]/DeviceCMYK"), "a radial DeviceN shading of Yellow and Ink");
+    assert!(pdf.contains("/C0[0 1]/C1[0.6 0]"), "the shading's tints: Yellow and Ink");
+    assert!(!warnings.iter().any(|w| w.contains("spot")), "{warnings:?}");
+}
+
 #[test]
 fn linked_stops_round_trip() {
     let (mut s, id) = setup();