run_sides

How many sides a primitive's run of vertices has (zero_run_sides(vc, wrap)), which vertex the end of a side lands on (zero_run_next(k, vc)), and ZERO_NONE, the value a slot holds when it names no edge or point (the largest i32, so it orders after every real index). A run of vc vertices has vc sides when it wraps and holds 3 or more, vc - 1 otherwise, and none below 2; side k joins vertex k to vertex zero_run_next(k, vc), the run's first after its last. The edges, edges_runs, loops and topology kernels walk runs with them, so all of them agree on what a face's perimeter is.