fix(cad): set the closed bit on DXF SPLINE entities so closed curves import into CAM (#1299)

A subpath that geometrically closes (e.g. an ellipse outline) was always emitted
with SPLINE group 70 = 8 (planar only). Autodesk's DXF reference defines bit 0
of group 70 as 'closed', and CAM software such as VCarve relies on it to treat
the cut as a pocket or profile rather than asking the operator to join endpoints.

Now `self.e.int(70, if sub.closed { 8 | 1 } else { 8 })`.

Test: extend lines_are_lwpolylines_curves_splines_and_fills_hatches to assert
the existing closed ellipse's SPLINE group 70 is '9' (1 closed + 8 planar).

crates/cad/src/scene.rs

@@ -652,8 +652,10 @@ impl<'a> Scene<'a> {
         let h = self.handle();
         self.e.entity("SPLINE", h, fixed::MODEL_RECORD, &layer, style, "AcDbSpline");
         self.e.normal();
-        // Planar.
-        self.e.int(70, 8);
+        // Planar (bit 3) and, if the subpath is closed, also closed (bit 0). A geometrically
+        // closed cut needs the closed flag set so CAM/CAM tools (e.g. VCarve) treat it as a
+        // pocket/profile rather than asking the operator to join endpoints. (#1299)
+        self.e.int(70, if sub.closed { 8 | 1 } else { 8 });
         self.e.int(71, 3);
         self.e.int(72, knots.len() as i64);
         self.e.int(73, ctrl.len() as i64);

crates/cad/src/tests.rs

@@ -155,6 +155,7 @@ fn lines_are_lwpolylines_curves_splines_and_fills_hatches() {
     // The ellipse: four cubics through clamped knots.
     let spline = &e[3];
     assert_eq!((get(spline, 71), get(spline, 73)), (Some("3"), Some("13")));
+    assert_eq!(get(spline, 70), Some("9"), "closed planar spline (#1299): 1 = closed, 8 = planar");
     assert_eq!(all(spline, 40), [0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 2.0, 2.0, 2.0, 3.0, 3.0, 3.0, 4.0, 4.0, 4.0, 4.0]);
     // R14 has lightweight polylines but no lineweights: the stroke is a polyline width.
     let r14 = export(&d, &DxfOptions { version: DxfVersion::R14, ..opts(&d) }).unwrap();