fix(spline): honour end tangents on a two-fit-point spline

fit_spline_polyline bailed to a straight chord below three fit points,
ignoring the stored begin/end tangents — a short tangent-clamped spline
rendered as a line. The C² solver handles n=2 as-is (both end rows are
clamp or natural conditions), so only the guard drops to n<2; tangentless
two-point splines still solve to the straight chord.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/entities/spline.rs

@@ -200,8 +200,7 @@ fn catmull_rom_polyline(pts: &[acadrust::types::Vector3], closed: bool) -> Vec<[
 fn fit_spline_polyline(spl: &Spline) -> Vec<[f64; 3]> {
     let p: Vec<[f64; 3]> = spl.fit_points.iter().map(|q| [q.x, q.y, q.z]).collect();
     let n = p.len();
-    if n < 3 {
-        // A single segment can't be C²-fit; a straight chord is the honest draw.
+    if n < 2 {
         return p;
     }