--smooth: turn grid staircases into octilinear runs after routing

A last stage on the finished result, wire by wire: a run of legs between
two points becomes the octilinear shape joining them when its copper is
clear, the furthest point first; right-angle corners left are chamfered;
straight runs are merged. Ends, vias, points where another wire of the net
meets the wire, and legs across own-net pads stay. Every new leg and corner
is checked on the exact shapes against other nets, pads, keepouts and the
board edge, slanted legs both ways round as the verifier counts a pair.
A net whose pads end up joined differently is put back, and the whole
smoothing is dropped if the finished check finds more violations than
before. --smooth-report adds track length per layer.

tests/test_cli.py

@@ -190,6 +190,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "window": ["--window-reroute", "1"],
         "vias": ["--reduce-vias"],
         "via_cost": ["--via-cost", "40"],
+        "smooth": ["--smooth"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: