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/doc/src/document.rs

@@ -1059,6 +1059,9 @@ pub enum FilletStyle {
     Constant,
     /// `radius` is the chord: the width across the blend.
     Chord,
+    /// G2 quintic blend of planar faces along a straight edge. `radius` sets the
+    /// equivalent circular setback, not a constant curvature radius.
+    Curvature,
     /// `radius` at the end of each edge nearest `start`, `radius2` at the other; `mid` radii at
     /// positions between (0 to 1 from the start end); straight between them, or `smooth`.
     Variable {
@@ -1653,7 +1656,7 @@ impl FeatureKind {
                         v.extend(mid.iter().map(|(_, r)| r.as_str()));
                     }
                     FilletStyle::Spline { radii, .. } => v.extend(radii.iter().map(String::as_str)),
-                    FilletStyle::Constant | FilletStyle::Chord => {}
+                    FilletStyle::Constant | FilletStyle::Chord | FilletStyle::Curvature => {}
                 }
             }
             FeatureKind::Chamfer { distance, distance2, angle, .. } => {