Sketch Blend Curve (tangent control-point spline between two curve ends) and the Centerline line type (sketch.centerline: left out of profiles, drawn as an axis)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/sketch/src/link.rs

@@ -286,7 +286,7 @@ impl Sketch {
                 LCurve::Conic(a, b, x, rho) => (CurveKind::Conic { a: point(self, a)?, b: point(self, b)?, apex: point(self, x)?, rho }, "k"),
             };
             let cid = self.fresh(prefix);
-            self.curves.push(Curve { id: cid, kind, construction: false, reversed: false, link: Some(id.to_string()) });
+            self.curves.push(Curve { id: cid, kind, construction: false, reversed: false, link: Some(id.to_string()), centerline: false });
         }
         for li in &lay.lone {
             point(self, *li)?;