test: failing regression test for #268: Panic from truck-topology lib.rs:186 when joining struts that end on a bore wall (#283)

* test: reproduce #268 (Panic from truck-topology lib.rs:186 when joining struts that end on a bore wall)

* Sew refuses edges whose ends weld to one vertex (#268)

The sewer welds vertices on a 1e-6 grid but only rejected edges shorter than
1e-9, so a sliver edge between the two welded its ends into one vertex and
truck-topology panicked ("Two same vertices cannot construct an edge") inside
the guard. Such an edge is now a degenerate-edge error before any truck call,
and the boolean takes its usual fallback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: bherbruck <brennen.herbruck@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/boolean_regression_tests.rs

@@ -64,3 +64,27 @@ fn oblique_cut_through_a_small_arc_matches_a_split() {
         assert!((got - above).abs() < 1e-6 * above, "{got} vs {above}");
     }
 }
+
+/// Joining struts that end on a bore wall reached a truck-topology panic (#268). The guard caught it,
+/// but no kernel call should panic at all (wasm aborts on a panic).
+#[test]
+fn struts_ending_on_a_bore_wall_join_without_a_panic() {
+    use std::sync::Arc;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+    let me = std::thread::current().id();
+    let count = Arc::new(AtomicUsize::new(0));
+    let c2 = count.clone();
+    let prev = std::panic::take_hook();
+    std::panic::set_hook(Box::new(move |_| {
+        if std::thread::current().id() == me {
+            c2.fetch_add(1, Ordering::SeqCst);
+        }
+    }));
+    let mut s = Session::default();
+    let script: Value = serde_json::from_str(include_str!("../../../examples/regressions/join_struts_on_bore_wall.json")).unwrap();
+    let result = s.run_script(&script);
+    std::panic::set_hook(prev);
+    assert_eq!(count.load(Ordering::SeqCst), 0, "kernel panics during the join");
+    result.unwrap();
+    assert_eq!(s.model.state().bodies.len(), 1);
+}