FacetCursor

A hasNext/vertex/index/advance() cursor over tau's facets, mirroring CofacetCursor above -- vertex is the vertex REMOVED to produce the facet at index. _index is iiB + iA (the OLD iA, before this step's own update), not iiB + iiA -- easy to invert by mistake, so kept exactly. Decodes tau via decodeToArray, not apply(...).toSeq.sorted (a Simplex[Int] decode followed by a redundant re-sort of an already-sorted SortedSet).