fix(kernel): fillet that uses up a side face makes a full round (#317)

A constant-radius fillet whose setback equals the width of a neighbouring
face was refused ("the blend is larger than the neighbouring faces"), so
r = 3 on the vertical edges of a 6 mm wide box failed while 2.999 worked.
The corner edge is now consumed: the rail ends at its far vertex and a side
face used up at both ends is removed, its remaining edge becoming the rail.
A face used up at one end only, and radii past the limit, are still refused.

Refs #199


Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests.rs

@@ -22,6 +22,17 @@ fn rel(a: f64, b: f64) -> f64 {
     (a - b).abs() / b.abs().max(1e-12)
 }
 
+#[test]
+fn fillet_as_wide_as_a_face_rounds_it_off() {
+    // #199: 14 x 6 x 10 box, the four vertical edges at r = 3 (half the 6 mm end faces).
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 14, "width": 6, "height": 10, "corner": [14, 0, 0]}));
+    run(&mut s, "solid.fillet", json!({"edges": [[14, 0, 5], [28, 0, 5], [14, 6, 5], [28, 6, 5]], "radius": "3"}));
+    let expect = 840.0 - (4.0 - PI) * 9.0 * 10.0;
+    assert!(rel(volume(&mut s), expect) < 2e-4, "{} vs {expect}", volume(&mut s));
+    assert!(s.execute("solid.fillet", &json!({"edges": [[21, 0, 10]], "radius": 3.5})).is_err());
+}
+
 #[test]
 fn join_extrude_from_a_chamfered_top_face() {
     let mut s = Session::default();