Add the command line entry point

python -m turtle_challenges <challenge> and the turtle-challenges command open a
window, run the named challenge and wait for a click. square and dashed-line are
registered; a missing Tk is reported with a hint instead of a traceback.

Task: MIL-002#6

tests/test_cli.py

@@ -0,0 +1,97 @@
+import pytest
+
+from tests.fakes import FakePen, FakeWindow
+from turtle_challenges import cli
+from turtle_challenges.constants import (
+    CHALLENGE_NAMES,
+    DASH_COUNT,
+    SQUARE_SIDES,
+)
+from turtle_challenges.pen import Pen, Window
+from turtle_challenges.window import TurtleUnavailableError
+
+
+@pytest.fixture
+def fake_pen_and_window(monkeypatch: pytest.MonkeyPatch) -> tuple[FakePen, FakeWindow]:
+    """Replace the turtle window and pen of the command line with fakes."""
+    pen = FakePen()
+    window = FakeWindow()
+
+    def create_fake_window() -> Window:
+        return window
+
+    def create_fake_pen() -> Pen:
+        return pen
+
+    monkeypatch.setattr(cli, "create_window", create_fake_window)
+    monkeypatch.setattr(cli, "create_pen", create_fake_pen)
+    return pen, window
+
+
+def test_every_challenge_name_has_a_challenge() -> None:
+    challenges = cli.build_challenges(FakePen())
+
+    assert set(challenges) == set(CHALLENGE_NAMES)
+
+
+@pytest.mark.parametrize("name", CHALLENGE_NAMES)
+def test_parser_accepts_every_challenge_name(name: str) -> None:
+    args = cli.build_parser().parse_args([name])
+
+    assert args.challenge == name
+
+
+def test_parser_rejects_an_unknown_challenge(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    with pytest.raises(SystemExit) as exit_info:
+        cli.build_parser().parse_args(["triangle"])
+
+    assert exit_info.value.code == 2
+    assert "invalid choice" in capsys.readouterr().err
+
+
+def test_help_lists_every_challenge(capsys: pytest.CaptureFixture[str]) -> None:
+    with pytest.raises(SystemExit) as exit_info:
+        cli.build_parser().parse_args(["--help"])
+
+    assert exit_info.value.code == 0
+    output = capsys.readouterr().out
+    assert all(name in output for name in CHALLENGE_NAMES)
+
+
+def test_main_runs_the_square_and_waits_for_a_click(
+    fake_pen_and_window: tuple[FakePen, FakeWindow],
+) -> None:
+    pen, window = fake_pen_and_window
+
+    status = cli.main(["square"])
+
+    assert status == 0
+    assert len(pen.args_of("forward")) == SQUARE_SIDES
+    assert window.waited_for_click
+
+
+def test_main_runs_the_dashed_line(
+    fake_pen_and_window: tuple[FakePen, FakeWindow],
+) -> None:
+    pen, _ = fake_pen_and_window
+
+    cli.main(["dashed-line"])
+
+    assert len(pen.args_of("forward")) == 2 * DASH_COUNT
+
+
+def test_main_reports_a_missing_turtle_and_exits_with_status_1(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    def fail() -> Window:
+        raise TurtleUnavailableError("no Tk here")
+
+    monkeypatch.setattr(cli, "create_window", fail)
+
+    with pytest.raises(SystemExit) as exit_info:
+        cli.main(["square"])
+
+    assert exit_info.value.code == 1
+    assert "no Tk here" in capsys.readouterr().err

tests/test_constants.py

@@ -0,0 +1,5 @@
+from turtle_challenges import constants
+
+
+def test_challenge_names_are_unique() -> None:
+    assert len(set(constants.CHALLENGE_NAMES)) == len(constants.CHALLENGE_NAMES)