Honour KiCad's keepouts: keyhole rings, wire_keepout, per-tile rasterizing

On the TerraHawk rc4 power stage every engine misread the board's rule areas:

- A no-track ring along the board edge arrives as one keyhole polygon, and
  keepout polygons were wrapped in a single convex tile, so the ring became
  the whole board. Split keepout polygons into convex pieces instead.
- The grid engine rasterized an area by its overall bounding box, which for
  that ring is again the whole board; it routed nothing. Rasterize per tile.
- "Keep router off" rule areas arrive as wire_keepout, which the reader did
  not know, so tracks ran through them (172 DRC items_not_allowed). Read it
  at board and image level. For now it blocks vias too, which is stricter
  than KiCad and cannot create a violation.
- place_keepout only restricts part placement; stop treating it as copper.

The oracle now runs the JAR headless; it was opening FreeRouting's GUI on
the desktop for every oracle test.

src/freeroute/route/grid_router.py

@@ -199,11 +199,14 @@ class GridRouter:
             elif isinstance(item, ObstacleArea):
                 if item.layer not in self.layers:
                     continue
-                box = item.bounding_box()
-                if box.is_empty():
-                    continue
-                for cell in self._cells_in_box(box, self.clearance):
-                    self._keepout.add((*cell, item.layer))
+                # Tile by tile: the area's own bounding box of a ring (a
+                # no-track border along the board edge) is the whole board.
+                for tile in item.tiles:
+                    box = tile.bounding_box()
+                    if box.is_empty():
+                        continue
+                    for cell in self._cells_in_box(box, self.clearance):
+                        self._keepout.add((*cell, item.layer))
 
     # --- occupancy cost -----------------------------------------------------