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/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)