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

src/turtle_challenges/__main__.py

@@ -0,0 +1,9 @@
+"""!
+@file __main__.py
+@brief Entry point of `python -m turtle_challenges`.
+"""
+
+from turtle_challenges.cli import main
+
+if __name__ == "__main__":
+    raise SystemExit(main())

src/turtle_challenges/cli.py

@@ -0,0 +1,74 @@
+"""!
+@file cli.py
+@brief Command line: run one challenge in a turtle window.
+"""
+
+import argparse
+from collections.abc import Callable, Sequence
+from functools import partial
+
+from turtle_challenges.constants import (
+    CHALLENGE_DASHED_LINE,
+    CHALLENGE_NAMES,
+    CHALLENGE_SQUARE,
+)
+from turtle_challenges.dashed_line import draw_dashed_line
+from turtle_challenges.pen import Pen
+from turtle_challenges.square import draw_square
+from turtle_challenges.window import (
+    TurtleUnavailableError,
+    create_pen,
+    create_window,
+)
+
+
+def build_parser() -> argparse.ArgumentParser:
+    """!
+    @brief Build the argument parser of the command line.
+
+    @return A parser with the challenge name.
+    """
+    parser = argparse.ArgumentParser(
+        prog="turtle-challenges",
+        description="Run one of the turtle challenges in a window. "
+        "Click the window to close it.",
+    )
+    parser.add_argument(
+        "challenge",
+        choices=CHALLENGE_NAMES,
+        help="the challenge to run",
+    )
+    return parser
+
+
+def build_challenges(pen: Pen) -> dict[str, Callable[[], None]]:
+    """!
+    @brief Bind every challenge to a turtle.
+
+    @param pen The turtle that draws.
+    @return A mapping from the command line name to a function without
+        arguments that draws the challenge.
+    """
+    return {
+        CHALLENGE_SQUARE: partial(draw_square, pen),
+        CHALLENGE_DASHED_LINE: partial(draw_dashed_line, pen),
+    }
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    """!
+    @brief Run the challenge named on the command line.
+
+    @param argv Arguments without the program name; `sys.argv` if omitted.
+    @return The exit status: 0 on success.
+    """
+    parser = build_parser()
+    args = parser.parse_args(argv)
+    try:
+        window = create_window()
+        tim = create_pen()
+    except TurtleUnavailableError as error:
+        parser.exit(1, f"{parser.prog}: error: {error}\n")
+    build_challenges(tim)[args.challenge]()
+    window.exitonclick()
+    return 0

src/turtle_challenges/constants.py

@@ -31,3 +31,15 @@ DASH_LENGTH = 10
 
 ## Challenge 2: length of the gap after each dash, in turtle units.
 GAP_LENGTH = 10
+
+## Command line name of challenge 1.
+CHALLENGE_SQUARE = "square"
+
+## Command line name of challenge 2.
+CHALLENGE_DASHED_LINE = "dashed-line"
+
+## Names of the challenges on the command line, in the order of the course.
+CHALLENGE_NAMES: tuple[str, ...] = (
+    CHALLENGE_SQUARE,
+    CHALLENGE_DASHED_LINE,
+)