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/format.rs

@@ -135,6 +135,9 @@
 
 //! - 19: editable associative offset-distance dimensions. Older offsets retain their saved
 //!   fixed distances; migration does not invent dimensions or change geometry.
+//!   Under the same version, a fillet's style can be `curvature` (a G2 quintic blend; `radius`
+//!   is the equivalent circular setback). Fillets of the other styles save as before; 0.9.0
+//!   would refuse the unknown style.
 
 use serde_json::Value;