[joseph-x-li]

Fixes #14.

edge_topology now checks the sorted half-edge list for any vertex pair that occurs
three or more times and returns
Err("Input mesh has an edge shared by more than two faces."), before the counting and
pairing loops that assume at most two. Valid (2-manifold or bordered) input takes the
same path as before; this only turns a panic into an error.

Adds edge_shared_by_four_faces_is_an_error_not_a_panic to src/tests.rs (two
tetrahedra sharing an edge), which panics on main and passes with this change.

Diff (src/manifold/hmesh.rs):

    ett.sort();

+   // An edge shared by more than two faces cannot be paired into halfedges; the
+   // counting and pairing below assume at most two and would index past the end.
+   if ett.windows(3).any(|w| w[0][..2] == w[1][..2] && w[1][..2] == w[2][..2]) {
+       return Err("Input mesh has an edge shared by more than two faces.".into());
+   }
+
    let mut ne = 1;

🤖 Generated with Claude Code

[komietty]

Thank you for your PR. LGTM.