Add item hierarchy, BasicBoard, and DSN->board construction

Ports the data model of the board Item hierarchy (Item base, Pin,
ObstacleArea/ConductionArea, Via, Trace) and BasicBoard (layers, nets,
clearance, bounding box, items with query helpers), then build_board:
the load-bearing integration that constructs a BasicBoard from a parsed
DsnBoard.

build_board maps layers -> LayerStructure, resolution -> transform,
default clearance rule -> ClearanceMatrix, nets -> Nets plus a
(component,pin)->net map, padstacks x placement -> Pin items, and
keepouts -> ObstacleArea items. Rectangle pads become exact IntBoxes,
convex polygon pads exact Simplexes, circle pads their bounding box
(documented approximation); every pad is centred on its pin location so
it contains that location by construction. Trace/Via are router-produced
and lightweight here (an imported unrouted board has none).

Validated on a real KiCad export (kicad_routable.dsn): layer/net/pin
counts match the parsed DSN, every pin's pad shape contains its origin,
every pin reports a valid net, and pin locations lie in the board
bounding box. An oracle-gated test cross-checks that every net the
reference FreeRouting JAR routes exists on the constructed board.

src/freeroute/board/__init__.py

@@ -0,0 +1,39 @@
+"""Board data model for freeroute.
+
+The data model a parsed DSN produces and the (future) router operates on:
+layers, units, coordinate transform, nets, clearance matrix, the item hierarchy,
+and the :class:`BasicBoard` container. :func:`build_board` constructs a board
+from a :class:`~freeroute.dsn.model.DsnBoard`.
+
+The routing operations (``RoutingBoard``) and the spatial ``ShapeSearchTree``
+belong to the router phase.
+"""
+
+from __future__ import annotations
+
+from .board import BasicBoard
+from .build import build_board
+from .clearance import ClearanceMatrix
+from .items import ConductionArea, Item, ObstacleArea, Pin, Trace, Via
+from .layer import Layer, LayerStructure
+from .net import Net, Nets
+from .transform import CoordinateTransform
+from .unit import Unit
+
+__all__ = [
+    "Unit",
+    "Layer",
+    "LayerStructure",
+    "CoordinateTransform",
+    "Net",
+    "Nets",
+    "ClearanceMatrix",
+    "Item",
+    "Pin",
+    "ObstacleArea",
+    "ConductionArea",
+    "Via",
+    "Trace",
+    "BasicBoard",
+    "build_board",
+]