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/ui-egui/src/dialogs.rs

@@ -689,7 +689,16 @@ impl Dialog {
             Kind::Extrude { to_offset, .. } if to_object_on(&self.inputs) => ("To Offset", ValueKind::Length, to_offset),
             Kind::Extrude { distance, .. } => ("Distance", ValueKind::Length, distance),
             Kind::Revolve { angle, .. } => ("Angle", ValueKind::Angle, angle),
-            Kind::Fillet { radius, chamfer, .. } => (if *chamfer { "Distance" } else { "Radius" }, ValueKind::Length, radius),
+            Kind::Fillet { radius, chamfer, .. } => {
+                let label = if *chamfer {
+                    "Distance"
+                } else if matches!(self.extra.get("type").and_then(Value::as_str), Some("curvature" | "g2")) {
+                    "Equivalent Radius"
+                } else {
+                    "Radius"
+                };
+                (label, ValueKind::Length, radius)
+            }
             Kind::Shell { thickness } => ("Thickness", ValueKind::Length, thickness),
             Kind::Draft { angle } => ("Angle", ValueKind::Angle, angle),
             Kind::Hole { diameter, .. } => ("Diameter", ValueKind::Length, diameter),
@@ -2059,7 +2068,13 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                                 &mut d.extra,
                                 "fi_type",
                                 "type",
-                                &[("Constant", "constant"), ("Chord Length", "chord"), ("Variable", "variable"), ("Spline", "spline")],
+                                &[
+                                    ("Constant", "constant"),
+                                    ("Chord Length", "chord"),
+                                    ("Variable", "variable"),
+                                    ("Spline", "spline"),
+                                    ("Curvature (G2)", "curvature"),
+                                ],
                             );
                             ui.end_row();
                         }
@@ -2069,6 +2084,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                                 match rtype {
                                     1 => "Chord Length",
                                     2 | 3 => "Start Radius",
+                                    4 => "Equivalent Radius",
                                     _ => "Radius",
                                 }
                             } else if *ctype == 0 {
@@ -4027,6 +4043,9 @@ pub fn for_feature(app: &SolveApp, id: u64, marker: Option<usize>) -> Option<Dia
             if let FeatureKind::Fillet { style, .. } = &f.kind {
                 match style {
                     FilletStyle::Constant => {}
+                    FilletStyle::Curvature => {
+                        d.extra.insert("type".into(), json!("curvature"));
+                    }
                     FilletStyle::Chord => {
                         d.extra.insert("type".into(), json!("chord"));
                     }
@@ -4043,7 +4062,7 @@ pub fn for_feature(app: &SolveApp, id: u64, marker: Option<usize>) -> Option<Dia
                                 d.extra.insert("knots".into(), json!(knots));
                                 d.extra.insert("radii".into(), json!(radii));
                             }
-                            FilletStyle::Constant | FilletStyle::Chord => {}
+                            FilletStyle::Constant | FilletStyle::Chord | FilletStyle::Curvature => {}
                         }
                         // The start was the first edge's last point: ends swapped.
                         let first_end = edges.first().and_then(|p| edge_sel(s, *p)).and_then(|x| match x {