Pin access: --access-cost, vias in pads inside wire keepouts, exact stub checks

--access-cost C makes a route pay C to pass where it would close another
pin's way out (its end cells, the via it may need there, and the
clearance around both) while that pin still has connections to route.

A pin inside a wire keepout can only leave by a via; its stub to the
cell centre was track inside the keepout, which the exact check refused.
The via now goes on the pin itself. Pin stubs are also checked exactly
against wire keepouts, as the exact engine will check them.

bench/diagnose.py reports what each refused connection collided with.

tests/test_cli.py

@@ -157,6 +157,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "seed": ["--seed", "3"],
         "orders": ["--orders", "2"],
         "promote": ["--promote-failures", "2"],
+        "access": ["--access-cost", "5"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: