--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.

src/freeroute/cli.py

@@ -30,8 +30,8 @@ from freeroute.ses import write_ses
 #: which optional flags each engine actually implements. Anything not listed for
 #: an engine is rejected by :func:`_validate` rather than quietly ignored.
 ENGINE_OPTIONS: dict[str, frozenset[str]] = {
-    "grid": frozenset({"rip_up", "max_passes", "seed", "orders", "promote", "access", "rollback"}),
-    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote", "access", "rollback"}),
+    "grid": frozenset({"rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback"}),
+    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote", "access", "negotiate", "rollback"}),
     "room": frozenset({"shove", "pack"}),
 }
 
@@ -46,6 +46,7 @@ _FLAG_NAMES = {
     "orders": "--orders",
     "promote": "--promote-failures",
     "access": "--access-cost",
+    "negotiate": "--negotiate",
     "rollback": "--no-rollback",
 }
 
@@ -167,6 +168,14 @@ def build_parser() -> argparse.ArgumentParser:
         help="grid and exact engines: extra cost for a route to pass where it would close "
         "another pin's way out while that pin still has connections to route (default 0: off)",
     )
+    parser.add_argument(
+        "--negotiate",
+        type=int,
+        metavar="N",
+        help="grid and exact engines: before routing, N rounds of negotiation on the empty "
+        "board, where routes may share space at a price that rises where they compete; "
+        "the contested cells then cost extra in the real routing (default 0: off)",
+    )
     parser.add_argument(
         "--no-rollback",
         dest="rollback",
@@ -212,6 +221,7 @@ def main(argv: list[str] | None = None) -> int:
             ("orders", args.orders is not None),
             ("promote", args.promote_failures is not None),
             ("access", args.access_cost is not None),
+            ("negotiate", args.negotiate is not None),
             ("rollback", not args.rollback),
         )
         if asked
@@ -247,6 +257,7 @@ def main(argv: list[str] | None = None) -> int:
             "resolution": 2 if args.grid_resolution is None else args.grid_resolution,
             "heuristic_weight": args.heuristic_weight,
             "access_cost": args.access_cost or 0.0,
+            "negotiate": args.negotiate or 0,
             "hole_to_hole_mm": args.hole_to_hole,
             "rollback": args.rollback,
         }
@@ -258,6 +269,7 @@ def main(argv: list[str] | None = None) -> int:
             "edge_clearance_mm": args.edge_clearance,
             "resolution": 1 if args.grid_resolution is None else args.grid_resolution,
             "access_cost": args.access_cost or 0.0,
+            "negotiate": args.negotiate or 0,
             "hole_to_hole_mm": args.hole_to_hole,
             "rollback": args.rollback,
         }