rejected by the exact check, unroutable even alone, walled in (a pin's
free region is closed off by other copper, with the nets that close it),
or congestion. The board state is the best pass's, rebuilt by replaying
its paths.
Runs every board with every variant under the same seeds and compares
them seed by seed, so a change is judged against the routing-order
noise rather than one lucky or unlucky run.
rsp2k