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/doc/src/assembly.rs

@@ -166,7 +166,13 @@ impl FeatureKind {
                 pt(origin);
                 dir(d);
             }
-            FeatureKind::Fillet { edges, .. } | FeatureKind::Chamfer { edges, .. } => edges.iter_mut().for_each(pt),
+            FeatureKind::Fillet { edges, style, .. } => {
+                edges.iter_mut().for_each(pt);
+                if let crate::FilletStyle::Variable { start, .. } = style {
+                    pt(start);
+                }
+            }
+            FeatureKind::Chamfer { edges, .. } => edges.iter_mut().for_each(pt),
             FeatureKind::Shell { faces, .. } | FeatureKind::OffsetFace { faces, .. } => faces.iter_mut().for_each(pt),
             FeatureKind::Draft { faces, neutral, pull, .. } => {
                 faces.iter_mut().for_each(pt);