Finer routing grid: --grid-resolution K with trace halos

A cell was exactly one trace plus its clearance, so a trace owning its cell
was the whole clearance model, and the 0.4 mm grid could not reach most
pins of a 0.5 mm pitch package: between two 0.3 mm pads a trace's centre
has a legal band only 0.15 mm wide. Resolution K makes cells K times
finer and has every trace claim a halo, the cells whose centre is nearer
than trace + clearance to one of its own, released again on rip-up.

K = 1 has an empty halo and is the old grid exactly (tested). On a 0.5 mm
row, K = 4 gets every pin out where K = 1 gets two. On the rc4 boards, K = 2
cuts the sense board's opens from 66 to 41 (JAR: 32) and the power board's
to 7 (JAR: 12), still with no DRC errors from freeroute's own copper.

src/freeroute/route/exact_router.py

@@ -92,6 +92,7 @@ def route_board_exact(
     shove_cap: int = 3,
     via_half: int | None = None,
     edge_clearance: int | None = None,
+    resolution: int = 1,
 ) -> ExactRouteResult:
     """Route ``board`` orthogonally and verify the result with exact geometry.
 
@@ -118,6 +119,7 @@ def route_board_exact(
         orthogonal=True,
         via_half=via_half,
         edge_clearance=edge_clearance,
+        resolution=resolution,
     )
     edge = 0 if edge_clearance is None else edge_clearance
     outline = board.outline