Route multi-pin nets as trees: spanning-tree pairs, T into routed copper

A net's pin groups were joined as a chain in pin order, and each
connection had to end on one chosen pin. Groups are now paired by a
minimum spanning tree (Manhattan distance between their nearest pins), and
a connection may start or end on any copper its net has already routed on
that side (path cells, via cells on every layer; not walled cells, whose
via moves into the pad). Plane nets keep their drops to the plane.

A connection that tees onto another records it as an anchor. Ripping a
connection rips what hangs on it, transitively, inside the same rip-up
transaction, so rollback restores all of them. The exact engine gets the
connections in commit order with their anchors and drops a T whose anchor
it rejects. --no-tree restores the chain.

tests/test_cli.py

@@ -160,6 +160,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "access": ["--access-cost", "5"],
         "negotiate": ["--negotiate", "2"],
         "rollback": ["--no-rollback"],
+        "tree": ["--no-tree"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: