Say which connections were not routed

freeroute exited 0 and said nothing when it dropped connections, which
cost real diagnosis time: on rc4 two simple two-pin nets came back with no
copper and no explanation. The grid now records each connection it could
not route, the exact check adds each it rejects, and the CLI lists them on
stderr (the SES is still written; the exit code stays 0, as the JAR's).

A connection whose ends both sit in one zone of its net is listed apart,
as left to the zone fill: freeroute does not route through a zone it
cannot trust to be solid, but KiCad's fill often joins those pads anyway.

src/freeroute/ses/model.py

@@ -57,6 +57,9 @@ class RoutingResult:
     """The complete routing output for a board, grouped by net."""
 
     nets: list[RoutedNet] = field(default_factory=list)
+    #: connections the router could not make: (net name, one end, other end,
+    #: the zone both ends sit in or "") -- a zone's fill may join the latter
+    unrouted: list[tuple[str, str, str, str]] = field(default_factory=list)
 
     def net(self, name: str, *, create: bool = False) -> RoutedNet | None:
         """Return the :class:`RoutedNet` with ``name`` (creating it if asked)."""