Add ShapeSearchTree: exact spatial index over item shapes

A bounding-box spatial hash (broad phase) with exact IntBox-intersection
narrow phase, replacing the exact router's occupancy grid. Supports
insert/remove-owner and overlap/region queries, a clearance_conflict test
(strict 2-D overlap of the clearance-expanded box, so tiles exactly a
clearance apart are allowed), and has_violation for the whole-board DRC
check. Tiles are tagged routed vs static so pad-vs-pad spacing in the
source design is not counted as a routing violation.

src/freeroute/board/__init__.py

@@ -17,6 +17,7 @@ from .clearance import ClearanceMatrix
 from .items import ConductionArea, Item, ObstacleArea, Pin, Trace, Via
 from .layer import Layer, LayerStructure
 from .net import Net, Nets
+from .search_tree import ShapeSearchTree, TreeShape
 from .transform import CoordinateTransform
 from .unit import Unit
 
@@ -35,5 +36,7 @@ __all__ = [
     "Via",
     "Trace",
     "BasicBoard",
+    "ShapeSearchTree",
+    "TreeShape",
     "build_board",
 ]