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/params.rs

@@ -77,6 +77,7 @@ impl FeatureKind {
             }
             FeatureKind::Fillet { radius, style, .. } => match style {
                 crate::FilletStyle::Constant => v.push(("Radius", radius, L)),
+                crate::FilletStyle::Curvature => v.push(("Equivalent radius", radius, L)),
                 crate::FilletStyle::Chord => v.push(("Chord length", radius, L)),
                 crate::FilletStyle::Variable { radius2, mid, .. } => {
                     v.push(("Start radius", radius, L));