Programmatic SPLINE limits the degree to 1..10 (fixes #268) (#269)

spline {control, degree} took any degree up to the control-point count
minus one; tessellation and extents grow with its square, so a few
thousand control points with a high degree hung the app on every redraw.
The Degree prompt, spline.cv and DXF input already limit it to 1..10.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/draw.rs

@@ -245,7 +245,9 @@ fn run_spline(s: &mut Session, p: &Value) -> Result<Value> {
         if c.len() < 2 {
             return Err(bad("spline", "need 2+ control points"));
         }
-        Spline::from_control(c, p.get("degree").and_then(Value::as_u64).unwrap_or(3) as usize)
+        // Degree 1..=10 as at the Degree prompt, `spline.cv` and DXF input (tessellation grows
+        // with its square).
+        Spline::from_control(c, p.get("degree").and_then(Value::as_u64).unwrap_or(3).clamp(1, 10) as usize)
     } else {
         return Err(bad("spline", "`fit` or `control` points are required"));
     };

crates/engine/tests/spline_degree.rs

@@ -0,0 +1,20 @@
+//! Programmatic SPLINE keeps the degree within 1..=10 like the Degree prompt (issue #268): a high
+//! degree on many control points made tessellation and extents take minutes.
+
+use cadcraft_engine::Session;
+use cadcraft_engine::doc::EntityKind;
+use serde_json::json;
+
+#[test]
+fn programmatic_spline_degree_is_limited() {
+    let control: Vec<[f64; 2]> = (0..3000).map(|i| [f64::from(i), f64::from(i % 7)]).collect();
+    for (degree, want) in [(json!(2_147_483_647), 10), (json!(2999), 10), (json!(11), 10), (json!(0), 1), (json!(4), 4)] {
+        let mut s = Session::new();
+        s.execute("spline", &json!({ "control": control, "degree": degree })).unwrap();
+        let d = s.doc().unwrap();
+        let Some(EntityKind::Spline(sp)) = d.model.iter().last().map(|e| &e.kind) else { panic!("no spline") };
+        assert_eq!(sp.degree, want);
+        let ext = d.extents(&s.space());
+        assert!(ext.min.x <= 0.0 && ext.max.x >= 2999.0, "{ext:?}");
+    }
+}