Manifold strange fin test case triggers assertion

[cakarsubasi]

When I ran tests on debug mode, I noticed that the following line of code triggered the debug assertion in merge_faces:

        // which we cannot merge since it would result in valence 1 vertices.
        bool can_we_merge = m.merge_faces(f, h0);
        CHECK_FALSE(can_we_merge);

    bool Manifold::merge_faces(FaceID f, HalfEdgeID h)
    {
        //assert that we're merging a valid face with the corresponding halfedge
        assert(kernel.face(h) == f);

Now, the test does expect merge_faces to fail but the failure behavior is inconsistent. So either:

1. Change assert to GEL_ASSERT and update test to use CHECK_THROWS
2. Replace initial assert with a check that returns false

[cakarsubasi]

Commit a8314e8 illustrates the issue.

https://github.com/cakarsubasi/GEL/actions/runs/16848371927/job/47731123769