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/kernel/src/sew.rs

@@ -363,7 +363,8 @@ fn sew_impl(faces: &[FaceSpec], touching: bool, sources: Option<&[Option<crate::
             }
             for e in l {
                 let (a, b) = e.ends();
-                if !a.is_finite() || !b.is_finite() || a.dist(b) < 1e-9 {
+                // Ends that weld to one vertex would make truck panic on a zero-length edge (#268).
+                if !a.is_finite() || !b.is_finite() || a.dist(b) < 1e-9 || key(a) == key(b) {
                     return Err(KernelError::Invalid("sew: a degenerate edge".into()));
                 }
             }