Route with the exact engine by default; warn when an engine does not check

The grid engine was the default and its copper is not checked against exact
shapes: a full reroute of the Labrador lid with no flags came out with 15
tracks_crossing and 87 clearance violations under KiCad's DRC while the
report listed only unrouted connections. The exact engine (what mckicad and
every benchmark already use) is now the default, and grid or room runs end
with a warning that their output needs a DRC.

tests/test_cli.py

@@ -71,13 +71,25 @@ def test_cli_every_engine_routes_a_fixture(engine, tmp_path):
     assert "(wire" in text
 
 
-def test_cli_default_engine_is_grid(tmp_path):
-    """The default must stay grid — the drop-in JAR replacement behaviour."""
+def test_cli_default_engine_is_exact(tmp_path):
+    """The default is the engine that checks its copper: grid output can short
+    and say nothing (a user routing with no flags shipped 15 crossings)."""
     default = tmp_path / "default.ses"
-    grid = tmp_path / "grid.ses"
+    exact = tmp_path / "exact.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()
+    assert main([str(FIXTURE), "--engine", "exact", "-do", str(exact)]) == 0
+    assert default.read_text() == exact.read_text()
+
+
+@pytest.mark.parametrize("engine", ["grid", "room"])
+def test_unchecked_engines_say_so(tmp_path, capsys, engine):
+    assert main([str(FIXTURE), "--engine", engine, "-do", str(tmp_path / "out.ses")]) == 0
+    assert "does not check its copper" in capsys.readouterr().err
+
+
+def test_the_exact_engine_does_not_warn(tmp_path, capsys):
+    assert main([str(FIXTURE), "-do", str(tmp_path / "out.ses")]) == 0
+    assert "WARNING" not in capsys.readouterr().err
 
 
 def test_cli_room_pack_routes(tmp_path):