nx_slice_contour_test.nx

nx_slice_contour_test.nx -- end-to-end: tetrahedron mesh -> BVH -> slice at midplane -> contour assembly. Verifies the 3 segments produced by nx_slice_plane_smoke get linked into exactly 1 closed polygon (the right triangle at z=0.5mm). Closed-form invariants: (a) Contours verdict == OK (every contour closed). (b) Exactly 1 polygon emitted. (c) n_open == 0. (d) Polygon has exactly 3 vertices (= 3 segments forming a closed triangle). (e) The vertex set equals {(0,0), (8192,0), (0,8192)} (any cyclic order accepted). (f) Polygon is well-formed: nx_polygon_signed_2area_q28 != 0 (non-degenerate triangle). expect_exit: 0 license_tier: ORIGINAL