CofacetCursor

A hasNext/vertex/index/advance() cursor over sigma's cofacets -- the enumeration cofacetIteratorWithVertex used to do directly as a hand-rolled Iterator[(Int, Long)], now factored out so a caller can read vertex/index as plain field accesses with zero per-step allocation, not even the (Int, Long) tuple Iterator[(Int, Long)]'s own contract forces on every next() call -- measured (.claude/WORKLOG-ripser-profiling.md's follow-up session) as ~49.8% of total allocation weight in both engines after every earlier fix in that arc, the largest remaining identified cost. hasNext/advance() are deliberately split from a single next(): vertex/index stay valid to re-read as many times as a caller wants between one advance() and the next (both PackedRipserCohomology.scala's coboundaryOf and Homology.scala's now read both fields off one candidate before advancing).

Decodes sigma via decodeToArray (a plain sorted Array[Int], binary-searched for membership), not apply (a Simplex[Int]/SortedSet[Int], O(log d) tree lookup per membership check) -- the same array-over-tree substitution decodeToArray's own doc motivates, folded in here since this cursor replaces cofacetIteratorWithVertex's body outright rather than wrapping it.