Variable-radius and chord-length fillets

New kernel file blend_variable.rs. The hooks into blend.rs are visibility
only: plane_normal, vtx, rebuild_face and unsupported are now pub(crate).
- fillet_variable(body, edge, start, r0, r1) works on the same edge
  configuration as the constant fillet (straight edge, planar faces, end
  faces square to it). The radius runs linearly from the end nearest
  `start`, and every section square to the edge is a circular arc. The
  blend is an exact degree (2, 1) rational B-spline: arc control points
  move linearly.
- fillet_chord(body, edges, chord): the radius per edge comes from its
  face angle (chord = 2 r cos(phi/2)).

Fillet feature: style constant | chord | variable {radius2, start};
FusionFilletEdgesCommand takes type, radius2 and start.

Tests:
- Variable 2 -> 5 on a box edge removes (1 - pi/4) L (r0^2 + r0 r1 + r1^2)/3.
  Starting from the other end gives the same solid; equal radii give the
  constant fillet; too large is refused.
- Chord 4 on a right angle equals radius 4/sqrt(2).
- Engine: both types by command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/blend.rs

@@ -21,16 +21,16 @@ enum Shape {
     Flat,
 }
 
-fn unsupported(msg: &str) -> KernelError {
+pub(crate) fn unsupported(msg: &str) -> KernelError {
     KernelError::Failed(format!("not supported yet: {msg}"))
 }
 
-fn vtx(v: &mt::Vertex) -> Vec3 {
+pub(crate) fn vtx(v: &mt::Vertex) -> Vec3 {
     from_p3(v.point())
 }
 
 /// Outward unit normal of a planar face (from its oriented surface), if planar.
-fn plane_normal(f: &mt::Face) -> Option<Vec3> {
+pub(crate) fn plane_normal(f: &mt::Face) -> Option<Vec3> {
     match f.oriented_surface() {
         mt::Surface::Plane(p) => {
             let n = p.normal();
@@ -42,7 +42,7 @@ fn plane_normal(f: &mt::Face) -> Option<Vec3> {
 
 /// Replace edges in a face's oriented boundary (by id, keeping orientation), and insert extra
 /// edges where consecutive edges no longer connect.
-fn rebuild_face(f: &mt::Face, subst: &HashMap<mt::EdgeID, mt::Edge>, connectors: &[mt::Edge]) -> Result<mt::Face> {
+pub(crate) fn rebuild_face(f: &mt::Face, subst: &HashMap<mt::EdgeID, mt::Edge>, connectors: &[mt::Edge]) -> Result<mt::Face> {
     let mut wires = Vec::new();
     for w in f.absolute_boundaries() {
         let mut edges: Vec<mt::Edge> = Vec::new();