Exact engine defaults to --grid-resolution 2

Resolution 2 reaches 0.5 mm pitch pins and cut unconnected items on both
rc4 boards; since lazy via checks it costs about 2x in time, not 4x.
The grid engine keeps 1, the old grid exactly.

src/freeroute/cli.py

@@ -113,10 +113,10 @@ def build_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         "--grid-resolution",
         type=int,
-        default=1,
+        default=None,
         metavar="K",
-        help="grid and exact engines: cells K times finer than trace + clearance "
+        help="grid and exact engines: cells K times finer than trace + clearance; "
-        "(default 1); 2 or more reaches fine-pitch pins, at some cost in time",
+        "default 2 for exact (reaches 0.5 mm pitch pins), 1 for grid",
     )
     parser.add_argument(
         "--heuristic-weight",
@@ -182,7 +182,7 @@ def main(argv: list[str] | None = None) -> int:
             rip_up=rip_up,
             max_passes=max_passes,
             edge_clearance_mm=args.edge_clearance,
-            resolution=args.grid_resolution,
+            resolution=2 if args.grid_resolution is None else args.grid_resolution,
             heuristic_weight=args.heuristic_weight,
         )
     elif args.engine == "room":
@@ -193,7 +193,8 @@ def main(argv: list[str] | None = None) -> int:
     else:
         result = build_routing_result(
             dsn, layers=layers, rip_up=rip_up, max_passes=max_passes,
-            edge_clearance_mm=args.edge_clearance, resolution=args.grid_resolution,
+            edge_clearance_mm=args.edge_clearance,
+            resolution=1 if args.grid_resolution is None else args.grid_resolution,
         )
 
     ses_text = write_ses(dsn, result)