Fix feather points when deleting first vertex of a closed mask path

Found while testing crates/keyframe/src/value.rs:79-95 (ShapePath::feather_after_remove, called from crates/engine/src/commands/mask.rs:330)

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

crates/keyframe/src/value.rs

@@ -82,7 +82,18 @@ impl ShapePath {
             if i == 0 && f.segment == 0 && !self.closed {
                 return false;
             }
-            if i > 0 && f.segment == i - 1 {
+            if i == 0 && self.closed && segs > 0 {
+                // The old closing segment and segment 0 merge into the new closing one.
+                if f.segment == segs {
+                    f.segment = segs - 1;
+                    f.t *= 0.5;
+                } else if f.segment == 0 {
+                    f.segment = segs - 1;
+                    f.t = 0.5 + f.t * 0.5;
+                } else {
+                    f.segment -= 1;
+                }
+            } else if i > 0 && f.segment == i - 1 {
                 f.t *= 0.5;
             } else if f.segment == i && i > 0 {
                 f.segment = i - 1;

crates/keyframe/tests/feather_remove.rs

@@ -0,0 +1,14 @@
+use effectcraft_keyframe::{FeatherPoint, ShapePath};
+
+#[test]
+fn removing_first_vertex_of_closed_path_merges_closing_segment() {
+    let mut p = ShapePath::rect([0.0, 0.0], 10.0, 10.0);
+    let fp = |segment| FeatherPoint { segment, t: 0.5, radius: 1.0, tension: 0.0 };
+    p.feather = vec![fp(3), fp(0)];
+    p.vertices.remove(0);
+    p.in_tangents.remove(0);
+    p.out_tangents.remove(0);
+    p.feather_after_remove(0);
+    let got: Vec<(usize, f64)> = p.feather.iter().map(|f| (f.segment, f.t)).collect();
+    assert_eq!(got, vec![(2, 0.25), (2, 0.75)]);
+}