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/kernel/src/lib.rs

@@ -94,7 +94,7 @@ mod tspline;
 #[cfg(test)]
 mod untouched_faces_tests;
 
-pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet, fillet_each, fillet_max, fillet_max_each};
+pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet, fillet_each, fillet_g2, fillet_max, fillet_max_each};
 pub use blend_corner::fillet_radii;
 pub use blend_variable::{RadiusSpline, Taper, fillet_chord, fillet_tapered, fillet_variable, fillet_variable_points, fillet_variable_spline, taper};
 pub use body::{Body, CapSignature, DISPLAY_ANGLE, EdgeInfo, FaceInfo, FacePaint, FaceSource, Paint};