Expose the exact and room engines in the CLI

The grid MVP was the only track reachable from the command line, so the
continuous room engine, channel packing, shove, and 45-degree routing were
unusable by anything shelling out to freeroute.

Add --engine {grid,exact,room} (default grid, unchanged behaviour) plus
--pack / --shove / --diagonal, and honour -mp on the tracks that implement
rip-up passes. Not every engine supports every pass; ENGINE_OPTIONS is the
matrix and an unsupported combination exits 2 with a message naming the
engines that do support the flag, rather than silently no-opping.

tests/test_cli.py

@@ -1,4 +1,9 @@
-"""Tests for the freeroute CLI (Specctra DSN in, SES out)."""
+"""Tests for the freeroute CLI (Specctra DSN in, SES out).
+
+Covers the engine selection contract: every engine must route a fixture from the
+command line, and every option the selected engine does not implement must fail
+loudly rather than being silently dropped.
+"""
 
 from __future__ import annotations
 
@@ -6,9 +11,13 @@ from pathlib import Path
 
 import pytest
 
-from freeroute.cli import main
+from freeroute.cli import ENGINE_OPTIONS, main
 
-FIXTURE = Path(__file__).parent / "dsn" / "fixtures" / "simple_2net.dsn"
+FIXTURES = Path(__file__).parent / "dsn" / "fixtures"
+FIXTURE = FIXTURES / "simple_2net.dsn"
+CHANNEL_PACK = FIXTURES / "channel_pack.dsn"
+SHOVE_NEEDED = FIXTURES / "shove_needed.dsn"
+DIAGONAL_WIN = FIXTURES / "diagonal_win.dsn"
 
 
 def test_cli_jar_compatible_flags_write_ses(tmp_path):
@@ -35,3 +44,114 @@ def test_cli_unreadable_input_returns_2(capsys):
     rc = main(["/nonexistent/board.dsn"])
     assert rc == 2
     assert "cannot read" in capsys.readouterr().err
+
+
+def test_cli_bad_layers_errors(capsys):
+    with pytest.raises(SystemExit):
+        main([str(FIXTURE), "--layers", "top,bottom"])
+    assert "--layers must be comma-separated integers" in capsys.readouterr().err
+
+
+# --- engine selection -------------------------------------------------------
+
+
+@pytest.mark.parametrize("engine", sorted(ENGINE_OPTIONS))
+def test_cli_every_engine_routes_a_fixture(engine, tmp_path):
+    out = tmp_path / f"{engine}.ses"
+    rc = main([str(FIXTURE), "--engine", engine, "-do", str(out)])
+    assert rc == 0
+    text = out.read_text()
+    assert text.startswith("(session")
+    assert "(wire" in text
+
+
+def test_cli_default_engine_is_grid(tmp_path):
+    """The default must stay grid — the drop-in JAR replacement behaviour."""
+    default = tmp_path / "default.ses"
+    grid = tmp_path / "grid.ses"
+    assert main([str(FIXTURE), "-do", str(default)]) == 0
+    assert main([str(FIXTURE), "--engine", "grid", "-do", str(grid)]) == 0
+    assert default.read_text() == grid.read_text()
+
+
+def test_cli_room_pack_routes(tmp_path):
+    out = tmp_path / "pack.ses"
+    rc = main([str(CHANNEL_PACK), "--engine", "room", "--pack", "-do", str(out)])
+    assert rc == 0
+    assert "(wire" in out.read_text()
+
+
+def test_cli_room_shove_routes(tmp_path):
+    out = tmp_path / "shove.ses"
+    rc = main([str(SHOVE_NEEDED), "--engine", "room", "--shove", "-do", str(out)])
+    assert rc == 0
+    assert "(wire" in out.read_text()
+
+
+def test_cli_exact_shove_routes(tmp_path):
+    out = tmp_path / "shove.ses"
+    rc = main([str(SHOVE_NEEDED), "--engine", "exact", "--shove", "-do", str(out)])
+    assert rc == 0
+    assert "(wire" in out.read_text()
+
+
+def test_cli_exact_diagonal_routes(tmp_path):
+    out = tmp_path / "diag.ses"
+    rc = main([str(DIAGONAL_WIN), "--engine", "exact", "--diagonal", "-do", str(out)])
+    assert rc == 0
+    assert "(wire" in out.read_text()
+
+
+def test_cli_max_passes_is_honoured_not_advisory(tmp_path):
+    out = tmp_path / "mp.ses"
+    rc = main([str(FIXTURE), "--engine", "exact", "-mp", "3", "-do", str(out)])
+    assert rc == 0
+    assert "(wire" in out.read_text()
+
+
+# --- unsupported combinations must error, never be silently ignored ---------
+
+
+@pytest.mark.parametrize(
+    ("argv", "flag"),
+    [
+        (["--engine", "grid", "--pack"], "--pack"),
+        (["--engine", "grid", "--shove"], "--shove"),
+        (["--engine", "grid", "--diagonal"], "--diagonal"),
+        (["--engine", "exact", "--pack"], "--pack"),
+        (["--engine", "room", "--diagonal"], "--diagonal"),
+        (["--engine", "room", "--no-rip-up"], "--no-rip-up"),
+        (["--engine", "room", "-mp", "5"], "-mp/--max-passes"),
+    ],
+)
+def test_cli_unsupported_option_errors(argv, flag, capsys):
+    with pytest.raises(SystemExit) as excinfo:
+        main([str(FIXTURE), *argv])
+    assert excinfo.value.code == 2
+    err = capsys.readouterr().err
+    assert flag in err
+    assert "not supported by --engine" in err
+    assert "supported by:" in err
+
+
+def test_cli_unknown_engine_errors(capsys):
+    with pytest.raises(SystemExit):
+        main([str(FIXTURE), "--engine", "quantum"])
+    assert "invalid choice" in capsys.readouterr().err
+
+
+def test_cli_supported_combinations_are_accepted(tmp_path):
+    """Every (engine, flag) pair the matrix claims to support must actually run."""
+    argv_for = {
+        "pack": ["--pack"],
+        "shove": ["--shove"],
+        "diagonal": ["--diagonal"],
+        "rip_up": ["--no-rip-up"],
+        "max_passes": ["-mp", "4"],
+    }
+    for engine, options in ENGINE_OPTIONS.items():
+        for option in options:
+            out = tmp_path / f"{engine}-{option}.ses"
+            rc = main([str(FIXTURE), "--engine", engine, *argv_for[option], "-do", str(out)])
+            assert rc == 0, f"{engine} + {option} failed"
+            assert out.read_text().startswith("(session")