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/engine/src/tests.rs

@@ -1058,6 +1058,25 @@ fn split_body_by_a_face() {
     assert!(s.execute("FusionSplitBodyCommand", &json!({"body": "Block", "tool": {"body": "Tool", "point": [100, 0, 0]}})).is_err());
 }
 
+/// Fillet types: chord length (radius from the face angle) and a radius varying along the
+/// edge; volumes from the removed material.
+#[test]
+fn chord_and_variable_fillets() {
+    let mut s = Session::default();
+    run(&mut s, "PrimitiveBox", json!({"length": 10, "width": 20, "height": 30, "body_name": "B"}));
+    run(&mut s, "FusionFilletEdgesCommand", json!({"edges": [[10, 10, 30]], "radius": 4, "type": "chord"}));
+    let r = 4.0 / 2f64.sqrt();
+    let v = solvecraft_kernel::measure(&s.world_state().body("B").unwrap().body).unwrap().volume;
+    assert!(rel(v, 6000.0 - (1.0 - PI / 4.0) * r * r * 20.0) < 1e-4, "{v}");
+    let mut s = Session::default();
+    run(&mut s, "PrimitiveBox", json!({"length": 10, "width": 20, "height": 30, "body_name": "B"}));
+    run(&mut s, "FusionFilletEdgesCommand", json!({"edges": [[10, 10, 30]], "radius": 2, "type": "variable", "radius2": 5, "start": [10, 0, 30]}));
+    let v = solvecraft_kernel::measure(&s.world_state().body("B").unwrap().body).unwrap().volume;
+    let removed = (1.0 - PI / 4.0) * 20.0 * (4.0 + 10.0 + 25.0) / 3.0;
+    assert!(rel(v, 6000.0 - removed) < 1e-4, "{v}");
+    assert!(s.execute("FusionFilletEdgesCommand", &json!({"edges": [[10, 10, 30]], "radius": 2, "type": "variable"})).is_err());
+}
+
 /// Components export as a STEP assembly: products, occurrences, placements.
 #[test]
 fn components_export_as_step_assembly() {