Find non-manifold vertices in parallel

[Fedr]

Before this PR, duplicateNonManifoldVertices inspected every vertex sequentially by walking the chains of triangles around it. Now a parallel pass classifies each vertex first, counting the chains of connected triangles around it (numChains) and the neighbor vertices passed more than once (numRepeatedVerts). The subsequent sequential pass skips every vertex where no duplication can be necessary — a single chain and no repeated neighbors, which is the vast majority in real meshes — as well as the vertices without incident triangles at all (e.g. outside of the given region).

Also added a test with a vertex surrounded by both a closed ring of triangles and a separate chain, pinning the case numChains == 2 with one closed chain.