Curvature-continuous (G2) fillets for straight planar edges (#862)

Fillet gets a Curvature (G2) radius type (type: "curvature", alias "g2"): a quintic blend whose size is the equivalent circular setback, for convex and concave straight edges between planar faces with a perpendicular planar end face at each end. Unsupported cases are refused rather than falling back to a circular fillet, and a blend that would cross a loop in its end face is refused. Persisted as a fillet style under format 19; labels in en/de/ja/ko/zh.

Co-authored-by: midasdf <midasdf@users.noreply.github.com>

crates/engine/src/cmd/edit_tests.rs

@@ -8,6 +8,28 @@ fn run(s: &mut Session, id: &str, params: Value) -> Value {
     s.execute(id, &params).unwrap_or_else(|e| panic!("{id} {params}: {e}"))
 }
 
+#[test]
+fn g2_fillet_survives_edit_undo_and_reopen() {
+    use solidcraft_doc::{Document, FeatureKind, FilletStyle};
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 20, "width": 20, "height": 20, "name": "Base"}));
+    run(&mut s, "solid.fillet", json!({"edges": [[20, 0, 10]], "radius": 2, "type": "g2", "name": "Smooth"}));
+    assert!(matches!(&s.doc.find_feature("Smooth").unwrap().kind, FeatureKind::Fillet { style: FilletStyle::Curvature, .. }));
+    let before = run(&mut s, "inspect.measure", json!({}))["total"]["volume_mm3"].as_f64().unwrap();
+    run(&mut s, "timeline.edit", json!({"feature": "Smooth", "set": {"radius": "3"}}));
+    let after = run(&mut s, "inspect.measure", json!({}))["total"]["volume_mm3"].as_f64().unwrap();
+    assert!(after < before);
+    run(&mut s, "edit.undo", json!({}));
+    let undo = run(&mut s, "inspect.measure", json!({}))["total"]["volume_mm3"].as_f64().unwrap();
+    assert!((undo - before).abs() < 1e-6);
+    run(&mut s, "edit.redo", json!({}));
+    let mut reopened = Session::new(Document::from_json(&s.doc.to_json()).unwrap());
+    assert!(matches!(&reopened.doc.find_feature("Smooth").unwrap().kind, FeatureKind::Fillet { style: FilletStyle::Curvature, .. }));
+    let volume = run(&mut reopened, "inspect.measure", json!({}))["total"]["volume_mm3"].as_f64().unwrap();
+    assert!((volume - after).abs() < 1e-6);
+    assert!(s.execute("solid.fillet", &json!({"edges": [[0, 0, 10]], "radius": 2, "type": "curvature", "fit": "reduce"})).is_err());
+}
+
 #[test]
 fn redefining_a_blend_replaces_persistent_edge_selection_and_survives_resize() {
     let mut s = Session::default();