--negotiate: PathFinder rounds on the empty board before routing

Every connection is routed N times on the empty board, where routes may
share cells at a price that doubles each round for every other net on a
cell, and a cell that stays shared gains history. The real routing then
pays that history on free cells, so the first routes stop taking the space
later routes (a fine-pitch part's escapes) have no way round. Off by
default.

The old history term never steered: it only accrued at rip-up fights,
after the first routes had claimed the contested space, and routed
connections are never revisited in later passes.

tests/test_cli.py

@@ -158,6 +158,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "orders": ["--orders", "2"],
         "promote": ["--promote-failures", "2"],
         "access": ["--access-cost", "5"],
+        "negotiate": ["--negotiate", "2"],
         "rollback": ["--no-rollback"],
     }
     for engine, options in ENGINE_OPTIONS.items():