Add draw_square and draw_dashed_line with tests

Challenge 1 draws four sides and turns right 90 degrees after each. Challenge 2 puts
the pen down for a dash and up for a gap and leaves it down at the end. Lengths and
counts come from constants.py; invalid values raise ValueError.

Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5

src/turtle_challenges/__init__.py

@@ -3,5 +3,15 @@
 @brief Solutions to the turtle graphics challenges of day 18 of Udemy's
 100 Days of Code.
 
-The challenges are added one milestone at a time; see the README.
+The challenges so far are @ref turtle_challenges.square.draw_square and
+@ref turtle_challenges.dashed_line.draw_dashed_line. Each takes the turtle as its
+first argument, so it can be tested without a window.
 """
+
+from turtle_challenges.dashed_line import draw_dashed_line
+from turtle_challenges.square import draw_square
+
+__all__ = [
+    "draw_dashed_line",
+    "draw_square",
+]

src/turtle_challenges/constants.py

@@ -13,3 +13,21 @@ WINDOW_TITLE = "Turtle Challenges"
 ## Largest value of one color channel; also the turtle color mode, so that
 ## colors can be given as RGB tuples from 0 to 255.
 COLOR_MODE = 255
+
+## Degrees in a right angle.
+RIGHT_ANGLE_DEGREES = 90
+
+## Challenge 1: number of sides of a square.
+SQUARE_SIDES = 4
+
+## Challenge 1: length of one side of the square, in turtle units.
+SQUARE_SIDE_LENGTH = 100
+
+## Challenge 2: number of dashes in the dashed line.
+DASH_COUNT = 50
+
+## Challenge 2: length of one dash, in turtle units.
+DASH_LENGTH = 10
+
+## Challenge 2: length of the gap after each dash, in turtle units.
+GAP_LENGTH = 10

src/turtle_challenges/dashed_line.py

@@ -0,0 +1,40 @@
+"""!
+@file dashed_line.py
+@brief Turtle Challenge 2: draw a dashed line.
+"""
+
+from turtle_challenges.constants import DASH_COUNT, DASH_LENGTH, GAP_LENGTH
+from turtle_challenges.pen import Pen
+
+
+def draw_dashed_line(
+    pen: Pen,
+    dash_count: int = DASH_COUNT,
+    dash_length: float = DASH_LENGTH,
+    gap_length: float = GAP_LENGTH,
+) -> None:
+    """!
+    @brief Draw a line of dashes: pen down for a dash, pen up for a gap.
+
+    The pen is down again when the function returns, so that the caller can
+    keep drawing.
+
+    @param pen The turtle that draws.
+    @param dash_count How many dashes to draw.
+    @param dash_length Length of one dash, in turtle units.
+    @param gap_length Length of the gap after each dash, in turtle units.
+    @exception ValueError If @p dash_count is negative, or a length is not
+        positive.
+    """
+    if dash_count < 0:
+        raise ValueError(f"dash_count must not be negative, got {dash_count}")
+    if dash_length <= 0 or gap_length <= 0:
+        raise ValueError(
+            f"lengths must be positive, got dash {dash_length} and gap {gap_length}"
+        )
+    for _ in range(dash_count):
+        pen.pendown()
+        pen.forward(dash_length)
+        pen.penup()
+        pen.forward(gap_length)
+    pen.pendown()

src/turtle_challenges/square.py

@@ -0,0 +1,26 @@
+"""!
+@file square.py
+@brief Turtle Challenge 1: draw a square.
+"""
+
+from turtle_challenges.constants import (
+    RIGHT_ANGLE_DEGREES,
+    SQUARE_SIDE_LENGTH,
+    SQUARE_SIDES,
+)
+from turtle_challenges.pen import Pen
+
+
+def draw_square(pen: Pen, side_length: float = SQUARE_SIDE_LENGTH) -> None:
+    """!
+    @brief Draw a square: move forward, turn right, four times.
+
+    @param pen The turtle that draws.
+    @param side_length Length of each side, in turtle units.
+    @exception ValueError If @p side_length is not positive.
+    """
+    if side_length <= 0:
+        raise ValueError(f"side_length must be positive, got {side_length}")
+    for _ in range(SQUARE_SIDES):
+        pen.forward(side_length)
+        pen.right(RIGHT_ANGLE_DEGREES)