Adopt the Scoreboard class and its tests

Copied unchanged from the base: class Scoreboard(Turtle) with update_scoreboard, increase_score and game_over. MIL-003 checks the values against the day-21 lectures.

Task: MIL-002#4
Closes #11

src/snake_game/scoreboard.py

@@ -0,0 +1,58 @@
+"""! @file
+@brief The scoreboard: the text at the top of the screen that shows the score.
+
+The class follows the day-21 lecture on inheritance: `Scoreboard` is a `Turtle`
+that writes text. Like `food`, this module needs `turtle` when it is imported;
+the game imports it only when it starts, and the tests install a fake `turtle`
+module first.
+"""
+
+from turtle import Turtle
+
+from snake_game.constants import (
+    GAME_OVER_POSITION,
+    GAME_OVER_TEXT,
+    SCORE_LABEL,
+    SCOREBOARD_ALIGNMENT,
+    SCOREBOARD_COLOR,
+    SCOREBOARD_FONT,
+    SCOREBOARD_POSITION,
+)
+
+
+class Scoreboard(Turtle):
+    """! @brief Shows the score as text at the top centre of the screen."""
+
+    def __init__(self) -> None:
+        """! @brief Set the scoreboard up and write the score 0."""
+        super().__init__()
+        ## @brief The number of foods the snake has eaten in this game.
+        self.score = 0
+        self.color(SCOREBOARD_COLOR)
+        self.penup()
+        self.hideturtle()
+        self.goto(SCOREBOARD_POSITION)
+        self.update_scoreboard()
+
+    def update_scoreboard(self) -> None:
+        """! @brief Wipe the old text and write the label and the score."""
+        self.clear()
+        self.write(
+            f"{SCORE_LABEL}{self.score}",
+            align=SCOREBOARD_ALIGNMENT,
+            font=SCOREBOARD_FONT,
+        )
+
+    def increase_score(self) -> None:
+        """! @brief Add 1 to the score and write it."""
+        self.score += 1
+        self.update_scoreboard()
+
+    def game_over(self) -> None:
+        """! @brief Write the game-over text at the centre of the screen.
+
+        The score stays where it is: the old text is not wiped. Automatic drawing is
+        off in the game, so the screen must be updated after this call.
+        """
+        self.goto(GAME_OVER_POSITION)
+        self.write(GAME_OVER_TEXT, align=SCOREBOARD_ALIGNMENT, font=SCOREBOARD_FONT)