feat: stabilize face attachments and closed spline workflows

crates/engine/src/cmd/sketch_import.rs

@@ -89,7 +89,7 @@ fn add_geometry(sk: &mut Sketch, geom: &[Geom2], f: &dyn Fn(Vec2) -> Vec2, k: f6
                         idx.push(i);
                     }
                 }
-                (idx.len() >= 2).then_some(CurveKind::Spline { pts: idx, control: *control, degree: *degree })
+                (idx.len() >= 2).then_some(CurveKind::Spline { pts: idx, control: *control, degree: *degree, periodic: false })
             }
             Geom2::Conic { a, apex, b, rho } => {
                 let (a, x, b) = (pt(sk, *a)?, pt(sk, *apex)?, pt(sk, *b)?);