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/cli.py

@@ -198,6 +198,20 @@ def main(argv: list[str] | None = None) -> int:
         )
 
     ses_text = write_ses(dsn, result)
+    missing = [u for u in result.unrouted if not u[3]]
+    in_zone = [u for u in result.unrouted if u[3]]
+    if missing:
+        # the SES is still written; say plainly what it is missing
+        print(f"freeroute: {len(missing)} connection(s) not routed:", file=sys.stderr)
+        for net, a, b, _zone in missing:
+            print(f"  {net}: {a} to {b}", file=sys.stderr)
+    if in_zone:
+        print(
+            f"freeroute: {len(in_zone)} more connection(s) left to a zone fill that may join them:",
+            file=sys.stderr,
+        )
+        for net, a, b, zone in in_zone:
+            print(f"  {net}: {a} to {b} (both in zone {zone})", file=sys.stderr)
 
     if args.output:
         with open(args.output, "w", encoding="utf-8") as f: