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 <[email protected]>

crates/doc/src/eval.rs

@@ -4144,6 +4144,7 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             let input = mb.traced();
             // The blended body, and the lineage of what the last blend started from.
             let (nb, from) = match &f.kind {
+                FeatureKind::Fillet { style: crate::FilletStyle::Curvature, .. } => (kernel::fillet_g2(&mb.body, &edges, r)?, input),
                 FeatureKind::Fillet { style: crate::FilletStyle::Chord, .. } => (kernel::fillet_chord(&mb.body, &edges, r)?, input),
                 FeatureKind::Fillet { style: crate::FilletStyle::Variable { radius2, start, mid, smooth }, .. } => {
                     let r2 = val(vals, radius2, Kind::Length)?;