feat: variable fillet radius may run down to zero at one end (#675)

A variable fillet's radius may run down to zero at one end, where the round closes to the corner.

crates/engine/src/tests.rs

@@ -1592,6 +1592,12 @@ fn chord_and_variable_fillets() {
     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("solid.fillet", &json!({"edges": [[10, 10, 30]], "radius": 2, "type": "variable"})).is_err());
+    // Down to zero at the far end: the round closes to the corner.
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 20, "height": 30, "body_name": "B"}));
+    run(&mut s, "solid.fillet", json!({"edges": [[10, 10, 30]], "radius": 4, "type": "variable", "radius2": 0, "start": [10, 0, 30]}));
+    let v = solidcraft_kernel::measure(&s.world_state().body("B").unwrap().body).unwrap().volume;
+    assert!(rel(v, 6000.0 - (1.0 - PI / 4.0) * 20.0 * 16.0 / 3.0) < 1e-4, "{v}");
     // A radius through a middle point (a user parameter), straight between: 2 → 4 → 2.
     let mut s = Session::default();
     run(&mut s, "solid.box", json!({"length": 10, "width": 20, "height": 30, "body_name": "B"}));