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.rs

@@ -821,7 +821,7 @@ fn input_keys(kind: &FeatureKind) -> Vec<(String, solidcraft_doc::expr::Kind)> {
                     *key = format!("fillet.control.{i}");
                 }
             }
-            solidcraft_doc::FilletStyle::Constant | solidcraft_doc::FilletStyle::Chord => {}
+            solidcraft_doc::FilletStyle::Constant | solidcraft_doc::FilletStyle::Chord | solidcraft_doc::FilletStyle::Curvature => {}
         }
     }
     keys