Implement the freeroute CLI (DSN in, SES out, JAR-flag-compatible)

The pyproject declared a freeroute script but freeroute.cli was missing, so the
entry point was broken. Add it: reads a Specctra .dsn, routes (rip-up on by
default), writes SES. Accepts FreeRouting-compatible -de/-do flags so it can be
dropped in wherever 'java -jar freerouting.jar' was called.

src/freeroute/cli.py

@@ -0,0 +1,66 @@
+"""freeroute command-line interface: Specctra DSN in, routed SES out.
+
+Flag-compatible with the FreeRouting JAR (``-de`` / ``-do``) so it can be
+dropped in wherever ``java -jar freerouting.jar`` was invoked.
+"""
+
+from __future__ import annotations
+
+import argparse
+import sys
+
+from freeroute.dsn.reader import parse_dsn
+from freeroute.route.pipeline import build_routing_result
+from freeroute.ses import write_ses
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(
+        prog="freeroute",
+        description="Native PCB autorouter — route a Specctra DSN and write an SES session file.",
+    )
+    parser.add_argument("dsn", nargs="?", help="input Specctra .dsn file")
+    parser.add_argument(
+        "-de", "--design", dest="design", help="input .dsn (FreeRouting-compatible alias)"
+    )
+    parser.add_argument(
+        "-do", "--output", dest="output", help="output .ses file (default: stdout)"
+    )
+    parser.add_argument(
+        "-mp",
+        "--max-passes",
+        type=int,
+        default=None,
+        help="accepted for FreeRouting CLI compatibility (advisory; freeroute self-bounds passes)",
+    )
+    parser.add_argument("--no-rip-up", action="store_true", help="disable rip-up-and-retry")
+    parser.add_argument("--layers", help="comma-separated signal layer indices to route on")
+    args = parser.parse_args(argv)
+
+    dsn_path = args.design or args.dsn
+    if not dsn_path:
+        parser.error("no input DSN (pass a path, or -de <file>)")
+
+    layers = [int(x) for x in args.layers.split(",")] if args.layers else None
+
+    try:
+        with open(dsn_path, encoding="utf-8") as f:
+            dsn_text = f.read()
+    except OSError as e:
+        print(f"freeroute: cannot read {dsn_path}: {e}", file=sys.stderr)
+        return 2
+
+    dsn = parse_dsn(dsn_text)
+    result = build_routing_result(dsn, layers=layers, rip_up=not args.no_rip_up)
+    ses_text = write_ses(dsn, result)
+
+    if args.output:
+        with open(args.output, "w", encoding="utf-8") as f:
+            f.write(ses_text)
+    else:
+        sys.stdout.write(ses_text)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())