Add exact-geometry (orthogonal, DRC-clean) routing track

Adds route/exact_router.py: routes with the grid track's rip-up/multi-layer/
via topology search in orthogonal mode (axis-aligned segments -> exact IntBox
copper), then verifies clearance exactly against a ShapeSearchTree. A net
whose copper would come closer than the clearance to any accepted item is
dropped, so the emitted geometry is DRC-clean by construction; endpoints stay
exactly on pads and vias at real layer transitions.

GridRouter gains a non-breaking orthogonal option (4-connected). The pipeline
gains an engine= selector: 'grid' (default, unchanged, highest coverage) or
'exact' (DRC-verified). The grid MVP stays the fallback.

On the designed boards the exact track routes every net DRC-clean; on the
real KiCad board it matches the grid's connectivity (4/4 multi-pin nets) and
is DRC-clean where the grid MVP's widened centrelines are not. Deferred:
shove, and 45-degree/IntOctagon trace caps.

Tests: the exact router's output has no clearance violation (exact IntBox
check) on every fixture; endpoints on pads; traces orthogonal; crossing uses
a via; and a direct comparison showing the exact track is clean where the
grid MVP violates on the real board. Oracle-gated parity for engine='exact'.

src/freeroute/route/__init__.py

@@ -14,8 +14,15 @@ multi-layer via search, and shove are deferred.
 
 from __future__ import annotations
 
+from .exact_router import ExactRouteResult, route_board_exact
 from .grid_router import GridRouter, RouteResult, route_board
-from .pipeline import build_routing_result, route, route_dsn_board
+from .pipeline import (
+    build_exact_routing_result,
+    build_routing_result,
+    route,
+    route_dsn_board,
+    route_dsn_board_exact,
+)
 
 __all__ = [
     "GridRouter",
@@ -24,4 +31,8 @@ __all__ = [
     "route",
     "route_dsn_board",
     "build_routing_result",
+    "ExactRouteResult",
+    "route_board_exact",
+    "build_exact_routing_result",
+    "route_dsn_board_exact",
 ]