Test #3084: test that Export for Screens JPGs open in every colour model

Exports RGB, CMYK and grey JPGs at qualities 100, 80 and 50, at 1x and 2x,
and decodes each one. It also checks that only the CMYK files leave out the
JFIF segment (#3140). Covers #2096 too.

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

crates/engine/src/cmd/fileio/tests.rs

@@ -914,3 +914,34 @@ fn read_path_for_a_named_importer_stops_at_the_limit() {
     assert!(e.to_string().contains("larger than") && e.to_string().contains("drawing.tuv"), "{e}");
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+/// #3084, #2096: Export for Screens writes JPGs a strict reader opens, in every colour model and at
+/// the qualities and scales the reports used. A four-component (CMYK) file has no JFIF segment,
+/// which describes only one- and three-component images (#3140); RGB and gray files keep it.
+#[test]
+fn export_for_screens_jpgs_open_in_every_colour_model() {
+    let mut s = session(100.0, 80.0, 1);
+    s.execute("paint.setFill", &json!({"color": "#cc3300"})).unwrap();
+    s.execute("shape.rectangle", &json!({"x": 10, "y": 10, "width": 60, "height": 40})).unwrap();
+    for (model, components) in [("rgb", 3), ("cmyk", 4), ("gray", 1)] {
+        for quality in [100, 80, 50] {
+            let r = s
+                .execute(
+                    "document.exportForScreens",
+                    &json!({"formats": [{"format": "jpg"}, {"format": "jpg", "scale": 2}], "settings": {"jpg": {"colorModel": model, "quality": quality}}}),
+                )
+                .unwrap();
+            let files = r["files"].as_array().unwrap();
+            assert_eq!(files.len(), 2, "{model} {quality}: {r}");
+            for (f, width) in files.iter().zip([100, 200]) {
+                let bytes = b64(f);
+                let what = format!("{model} q{quality} {}", f["name"]);
+                assert_eq!(bytes.get(..2), Some(&[0xFF, 0xD8][..]), "{what}: starts with SOI");
+                let jfif = bytes.get(2..4) == Some(&[0xFF, 0xE0][..]);
+                assert_eq!(jfif, components != 4, "{what}: JFIF only on one- and three-component files");
+                let image = image::load_from_memory(&bytes).unwrap_or_else(|e| panic!("{what}: {e}"));
+                assert_eq!(image.width(), width, "{what}");
+            }
+        }
+    }
+}