--window-reroute N: route a finer grid around each connection still stuck

After the exact engine, every net routed wholly inside a window around a
stuck connection's pins is ripped, the rest of the board is laid down as
fixed copper, and the window alone is routed again at --window-resolution
(default 4) with the stuck net and the ripped nets only. The result is kept
only if fewer connections are left unrouted; each round doubles the margin.
The grid and exact routers gain only_nets and bounds for it. Off by default.

tests/test_cli.py

@@ -161,6 +161,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "negotiate": ["--negotiate", "2"],
         "rollback": ["--no-rollback"],
         "tree": ["--no-tree"],
+        "window": ["--window-reroute", "1"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: