relation: clear digraph scratch pointers

[ydah]

relation_digraph() stores the input bitset vector in the static F pointer and updates the pointed-to bitsets directly while traversing the relation.

The final function = F assignment only writes to a by-value parameter and therefore has no effect on the caller. Remove this dead assignment.

Also clear indexes and vertices after freeing them, and clear the borrowed R and F pointers before returning, so the static state does not retain stale or dangling pointers.

The transitive-closure behavior is unchanged because the caller's bitsets are modified in place.