Add MVP grid maze router and DSN->SES routing pipeline

Implements a working autorouter and the Java-free replacement for the
freerouting.jar step: dsn_text -> parse_dsn -> build_board -> route ->
write_ses.

GridRouter is a single-layer A* maze search over a uniform occupancy
grid: a cell is blocked by another net's pad (inflated by clearance +
half trace width) or a keepout; each net's ratsnest is connected pin to
pin; a routed trace then blocks other nets. The cell path becomes a
trace polyline whose endpoints are the exact pin locations. Board-unit
paths are converted back to DSN units for the SES (wire (path ...))
scopes.

This is an MVP, not a port of FreeRouting's expansion-room maze:
free-space rooms, rip-up-and-retry, multi-layer via search, and shove
are deferred (they raise quality/coverage, not the connectivity
milestone). It reaches connectivity on boards whose nets route on one
layer without crossing.

Adds tests/dsn/fixtures/simple_2net.dsn (a guaranteed-routable 2-net
board). Invariant tests: both nets route, the emitted SES parses, every
trace stays on a valid layer and within the board outline, and every
routed net's trace endpoints sit exactly on its two pads. An oracle-gated
test asserts connectivity parity with the reference FreeRouting JAR on
the simple board.

src/freeroute/route/__init__.py

@@ -0,0 +1,27 @@
+"""Autorouting for freeroute.
+
+An MVP grid-based maze router and the DSN-in / SES-out pipeline that makes
+freeroute a Java-free replacement for the ``freerouting.jar`` step::
+
+    from freeroute.route import route
+    ses_text = route(dsn_text)
+
+The router (:class:`GridRouter`) is intentionally simple — single-layer A* over
+a uniform occupancy grid, enough to reach connectivity on boards whose nets
+route without crossing. FreeRouting's free-space expansion-room maze, rip-up,
+multi-layer via search, and shove are deferred.
+"""
+
+from __future__ import annotations
+
+from .grid_router import GridRouter, RouteResult, route_board
+from .pipeline import build_routing_result, route, route_dsn_board
+
+__all__ = [
+    "GridRouter",
+    "RouteResult",
+    "route_board",
+    "route",
+    "route_dsn_board",
+    "build_routing_result",
+]