fix(color): mirror gradient midpoints when reversing

Move and mirror segment midpoints when reversing gradient stops so reversed gradients preserve color and opacity interpolation.

Validated locally on macOS with all six cargo xtask ci gates (1,277 tests passed, four existing tests ignored, 15 WASM compilation checks), plus independent regressions, undo/redo, native and IDML round trips, and PNG/PDF rendering.

Refs #638

crates/color/src/gradient.rs

@@ -104,9 +104,12 @@ impl Gradient {
         out
     }
     pub fn reverse(&mut self) {
+        // A midpoint belongs to the segment after its stop, so it moves with that segment and mirrors.
+        let mids: Vec<_> = self.stops.iter().rev().skip(1).map(|s| 1.0 - s.midpoint).collect();
         self.stops.reverse();
-        for s in &mut self.stops {
+        for (s, m) in self.stops.iter_mut().zip(mids.into_iter().chain([0.5])) {
             s.offset = 1.0 - s.offset;
+            s.midpoint = m;
         }
     }
     pub fn sort(&mut self) {
@@ -219,8 +222,24 @@ mod tests {
     #[test]
     fn reverse_stops() {
         let mut g = Gradient::default();
+        g.stops[0].midpoint = 0.25;
         g.reverse();
         assert_eq!(g.stops[0].color.to_hex(), "#000000");
         assert_eq!(g.stops[0].offset, 0.0);
+        // The midpoint moves to its reversed segment and mirrors.
+        assert!((g.sample(0.75).0.to_rgb()[0] - 0.5).abs() < 1e-5);
+        assert_eq!(g.stops.iter().map(|s| s.midpoint).collect::<Vec<_>>(), [0.75, 0.5]);
+
+        // With several stops, the reversed gradient is the original mirrored.
+        let mut g = Gradient::default();
+        g.stops[0].midpoint = 0.3;
+        g.stops.insert(1, GradientStop { offset: 0.4, color: Color::rgb(1.0, 0.0, 0.0), opacity: 0.5, midpoint: 0.8 });
+        let original = g.clone();
+        g.reverse();
+        for t in [0.1, 0.3, 0.5, 0.7, 0.9] {
+            let ((c, o), (want_c, want_o)) = (g.sample(t), original.sample(1.0 - t));
+            assert!(c.to_rgb().iter().zip(want_c.to_rgb()).all(|(a, b)| (a - b).abs() < 1e-5), "colour at {t}");
+            assert!((o - want_o).abs() < 1e-5, "opacity at {t}");
+        }
     }
 }