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/route/pipeline.py

@@ -82,6 +82,7 @@ def route_dsn_board(
     max_passes: int = 10,
     seed: int | None = None,
     route_first: frozenset | None = None,
+    access_cost: float = 0.0,
 ) -> tuple[RouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard`; return the board-unit result, the scale,
     and the layer names. ``layers`` restricts routing to those signal-layer
@@ -104,6 +105,7 @@ def route_dsn_board(
         max_passes=max_passes,
         seed=seed,
         route_first=route_first,
+        access_cost=access_cost,
     )
     return result, scale, [layer.name for layer in dsn.layers]
 
@@ -146,12 +148,14 @@ def build_routing_result(
     edge_clearance_mm: float | None = None,
     seed: int | None = None,
     route_first: frozenset | None = None,
+    access_cost: float = 0.0,
 ) -> RoutingResult:
     """Route ``dsn`` (grid track) and convert to a DSN-unit RoutingResult."""
     route, scale, layer_names = route_dsn_board(
         dsn, layers=layers, rip_up=rip_up, max_passes=max_passes,
         edge_clearance_mm=edge_clearance_mm, resolution=resolution, seed=seed,
         route_first=route_first,
+        access_cost=access_cost,
     )
     return _to_routing_result(route, dsn, scale, layer_names)
 
@@ -169,6 +173,7 @@ def route_dsn_board_exact(
     diagonal: bool = False,
     seed: int | None = None,
     route_first: frozenset | None = None,
+    access_cost: float = 0.0,
 ) -> tuple[ExactRouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard` with the exact-geometry (orthogonal,
     DRC-verified) track; return the exact result, the scale, and layer names.
@@ -195,6 +200,7 @@ def route_dsn_board_exact(
         diagonal=diagonal,
         seed=seed,
         route_first=route_first,
+        access_cost=access_cost,
     )
     return exact, scale, [layer.name for layer in dsn.layers]
 
@@ -212,6 +218,7 @@ def build_exact_routing_result(
     edge_clearance_mm: float | None = None,
     seed: int | None = None,
     route_first: frozenset | None = None,
+    access_cost: float = 0.0,
 ) -> RoutingResult:
     """Route ``dsn`` (exact track) and convert to a DSN-unit RoutingResult."""
     exact, scale, layer_names = route_dsn_board_exact(
@@ -226,6 +233,7 @@ def build_exact_routing_result(
         heuristic_weight=heuristic_weight,
         seed=seed,
         route_first=route_first,
+        access_cost=access_cost,
     )
     return _to_routing_result(exact.result, dsn, scale, layer_names)