M2.1: blend Align-to-Path smooth on curved spines

Spine::at_length computed tangents from flattened polyline chords, which
are piecewise constant and jump discretely between chords on curved
spines. Steps oriented to the spine with Align to Path stuttered at
large step counts.

Fix: store the original PathData in Spine and compute the tangent from
the original cubic at the true arc-length position via ParamCurveDeriv,
proportionally mapping flattened arc-length s onto each segment's curve
arclength using anchor_length values, instead of using piecewise-constant
chord directions.

Added missing docstrings for Spine::new and Spine::length to satisfy
docstring coverage requirements.

Co-authored-by: Craft Bot <bot@storytold.dev>

crates/doc/src/blend.rs

@@ -908,6 +908,8 @@ pub struct Spine {
 }
 
 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.
     pub fn new(path: &PathData) -> Option<Self> {
         let sp = path.subpaths.first()?;
         let mut pts: Vec<Point> = Vec::new();
@@ -946,6 +948,7 @@ impl Spine {
         }
         Some(Self { pts, cum, anchors, closed: sp.closed, path: path.clone() })
     }
+    /// The spine's flattened length in arc length.
     pub fn length(&self) -> f64 {
         *self.cum.last().unwrap_or(&0.0)
     }