Adds crossing_2net.dsn: NET_A is a full-width horizontal wall and NET_B's
pins sit above and below it, so single-layer routing cannot connect NET_B
but two-layer routing succeeds by dipping to the other layer through vias.
Tests assert the multi-layer invariants: single-layer routing leaves a net
unrouted with no vias; multi-layer connects both nets with >= 1 via; the
SES contains a via scope; every via sits at a real layer transition of its
net's path; trace/via endpoints still land on the pads; traces stay on
valid layers and within the outline; and no two different nets cross on the
same layer. Oracle-gated parity tests cover the simple and crossing boards.
Ryan Malloy
49c258e81d