Blend: the spine keeps only the subpath it samples

Spine stored a full clone of the blend's PathData but only ever read its
first subpath. Store that SubPath instead, which drops the extra
allocation and makes the field's purpose clear. Document the field and
the two cases where Spine::new returns None.

crates/doc/src/blend.rs

@@ -904,12 +904,16 @@ pub struct Spine {
     cum: Vec<f64>,
     anchors: Vec<f64>,
     closed: bool,
-    path: PathData,
+    /// The subpath the spine was flattened from, kept so [`Spine::at_length`] can read the
+    /// original curves' tangents. Only the first subpath is stored: the others never affect a
+    /// spine.
+    subpath: SubPath,
 }
 
 impl Spine {
     /// Build a spine from the first subpath of `path`: flatten it to a polyline, record cumulative
-    /// arc lengths, and the arc length at each anchor.
+    /// arc lengths, and the arc length at each anchor. `None` when `path` has no subpath, or when
+    /// the first one flattens to fewer than two points (so it has no length to walk).
     pub fn new(path: &PathData) -> Option<Self> {
         let sp = path.subpaths.first()?;
         let mut pts: Vec<Point> = Vec::new();
@@ -946,7 +950,7 @@ impl Spine {
         if pts.len() < 2 {
             return None;
         }
-        Some(Self { pts, cum, anchors, closed: sp.closed, path: path.clone() })
+        Some(Self { pts, cum, anchors, closed: sp.closed, subpath: sp.clone() })
     }
     /// The spine's flattened length in arc length.
     pub fn length(&self) -> f64 {
@@ -1012,7 +1016,7 @@ impl Spine {
     /// `Align to Path` jump between chords instead of following the curve smoothly.
     pub fn at_length(&self, s: f64) -> (Point, f64) {
         let s = if s.is_finite() { s.clamp(0.0, self.length()) } else { 0.0 };
-        let Some(sp) = self.path.subpaths.first() else { return (self.pts.first().copied().unwrap_or_default(), 0.0) };
+        let sp = &self.subpath;
         let n = sp.segment_count();
         let mut start = 0.0;
         for i in 0..n {