Route wider net classes at their width, falling back where they don't fit

The grid is laid out for the default width; a wider trace also needs the
cells its extra half width reaches to be free, and claims a wider halo.
A connection that does not fit at its class width (typically at a
fine-pitch pin) is routed at the default instead and reported. A net is
written at its class width when all of its connections went in at it,
and the exact check verifies it at that width; nets that shove or the
diagonal pass reshaped go out at the default.

tests/route/test_net_widths.py

@@ -0,0 +1,42 @@
+"""Net classes wider than the default are routed at their width where it fits."""
+
+from __future__ import annotations
+
+from freeroute.dsn import parse_dsn
+from freeroute.route.pipeline import build_exact_routing_result
+
+_BOARD = """(pcb widths (parser) (resolution um 10) (unit um)
+  (structure
+    (layer F.Cu (type signal))
+    (boundary (path pcb 0  0 0  20000 0  20000 -10000  0 -10000  0 0))
+    {keepouts}
+    (rule (width 150) (clearance 150)))
+  (placement (component S (place P1 2000 -5000 front 0) (place P2 18000 -5000 front 0)))
+  (library (image S (pin pad 1 0 0)) (padstack pad (shape (rect F.Cu -600 -600 600 600)) (attach off)))
+  (network (net PWR (pins P1-1 P2-1))
+    (class Power PWR (rule (width 400) (clearance 150)))))"""
+
+# two keepouts leave a 0.65 mm channel: room for 0.15 mm (0.45), not for 0.4 mm (0.7)
+_CHANNEL = (
+    '(keepout "" (polygon F.Cu 0  9000 0  11000 0  11000 -4675  9000 -4675))\n'
+    '    (keepout "" (polygon F.Cu 0  9000 -5325  11000 -5325  11000 -10000  9000 -10000))'
+)
+
+
+def _route(keepouts=""):
+    return build_exact_routing_result(parse_dsn(_BOARD.format(keepouts=keepouts)), resolution=2)
+
+
+def test_a_wider_class_is_routed_and_written_at_its_width():
+    result = _route()
+    assert result.unrouted == [] and result.narrowed == []
+    widths = {w.width for net in result.nets for w in net.wires}
+    assert widths == {400.0}
+
+
+def test_a_wider_class_that_does_not_fit_falls_back_to_the_default():
+    result = _route(_CHANNEL)
+    assert result.unrouted == []
+    assert [net for net, _a, _b in result.narrowed] == ["PWR"]
+    widths = {w.width for net in result.nets for w in net.wires}
+    assert widths == {150.0}