#1007: Reverse Gradient mirrors off-centre midpoints

Reversal read the midpoints after Gradient::reverse had reordered the stops, so each stop
took the wrong segment's midpoint: red-to-blue with its midpoint at 20% reversed to a 50%
midpoint instead of 80%, moving the transition.

Gradient::reverse now mirrors each segment's midpoint with it, so a reversed gradient is the
same ramp the other way round; paint.editGradient's own (wrong) pass is gone, and the SVG
importer's reflect spread, which had a correct copy of the same mirroring, uses it too.

Tests: two- and three-stop gradients with off-centre midpoints sample the same reversed (t vs
1 - t) and come back after two reversals; paint.editGradient reverse turns a 0.2 midpoint into
0.8.

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

# Conflicts:
#	ROADMAP.md

crates/engine/src/cmd/gradient.rs

@@ -270,13 +270,8 @@ pub(crate) fn apply_gradient_edit_in(
         gp.swatch = None;
     }
     if bool_or(p, "reverse", false) {
+        // Midpoints mirrored with their segments.
         gp.gradient.reverse();
-        // Midpoints belong to the segment to the right; mirror them.
-        let n = gp.gradient.stops.len();
-        let mids: Vec<f32> = gp.gradient.stops.iter().map(|s| s.midpoint).collect();
-        for i in 0..n {
-            gp.gradient.stops[i].midpoint = if i + 1 < n { 1.0 - mids[n - 2 - i] } else { 0.5 };
-        }
     }
     // Angle and aspect edits keep the focal point in its place in the extent ellipse.
     let before = gp.geom;