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.

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"}),
-    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote"}),
+    "grid": frozenset({"rip_up", "max_passes", "seed", "orders", "promote", "access"}),
+    "exact": frozenset({"shove", "diagonal", "rip_up", "max_passes", "seed", "orders", "promote", "access"}),
     "room": frozenset({"shove", "pack"}),
 }
 
@@ -45,6 +45,7 @@ _FLAG_NAMES = {
     "seed": "--seed",
     "orders": "--orders",
     "promote": "--promote-failures",
+    "access": "--access-cost",
 }
 
 
@@ -158,6 +159,13 @@ def build_parser() -> argparse.ArgumentParser:
         help="grid and exact engines: up to R rounds, each routing the connections that "
         "failed so far first; keeps the best round",
     )
+    parser.add_argument(
+        "--access-cost",
+        type=float,
+        metavar="C",
+        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(
         "--jobs",
         type=int,
@@ -186,6 +194,7 @@ def main(argv: list[str] | None = None) -> int:
             ("seed", args.seed is not None),
             ("orders", args.orders is not None),
             ("promote", args.promote_failures is not None),
+            ("access", args.access_cost is not None),
         )
         if asked
     ]
@@ -219,6 +228,7 @@ def main(argv: list[str] | None = None) -> int:
             "edge_clearance_mm": args.edge_clearance,
             "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,
         }
     elif args.engine == "grid":
         options = {
@@ -227,6 +237,7 @@ def main(argv: list[str] | None = None) -> int:
             "max_passes": max_passes,
             "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,
         }
     if args.engine == "room":
         result = build_rooms_routing_result(