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.

tests/test_cli.py

@@ -0,0 +1,37 @@
+"""Tests for the freeroute CLI (Specctra DSN in, SES out)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from freeroute.cli import main
+
+FIXTURE = Path(__file__).parent / "dsn" / "fixtures" / "simple_2net.dsn"
+
+
+def test_cli_jar_compatible_flags_write_ses(tmp_path):
+    out = tmp_path / "out.ses"
+    rc = main(["-de", str(FIXTURE), "-do", str(out)])
+    assert rc == 0
+    text = out.read_text()
+    assert text.startswith("(session")
+    assert "(net" in text and "(wire" in text
+
+
+def test_cli_positional_input_to_stdout(capsys):
+    rc = main([str(FIXTURE)])
+    assert rc == 0
+    assert "(session" in capsys.readouterr().out
+
+
+def test_cli_missing_input_errors():
+    with pytest.raises(SystemExit):
+        main([])
+
+
+def test_cli_unreadable_input_returns_2(capsys):
+    rc = main(["/nonexistent/board.dsn"])
+    assert rc == 2
+    assert "cannot read" in capsys.readouterr().err