Adopt constants.py and its test

Copied from the base with identity-only edits: the docstrings name days 20 and 21, and the package main page describes the whole game. The values are unchanged; MIL-003 checks them against the day-21 lectures.

Task: MIL-001#3
Closes #3

src/snake_game/__init__.py

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+## @package snake_game
+#  @brief Snake game from Udemy's 100 Days of Code (day 21).
+#
+#  @mainpage Snake Game
+#
+#  A snake of three square segments moves across a Python turtle screen by
+#  itself and is steered with the arrow keys. It eats food, grows and keeps a
+#  score, and the game is over when the head passes the wall or touches the
+#  tail. The game uses the standard library only.
+#
+#  The constants of the game are in constants.py.
+
+"""Snake game from Udemy's 100 Days of Code (day 21)."""

src/snake_game/constants.py

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+## @file constants.py
+#  @brief Constants of the snake game.
+#
+#  Every value that the day-20 and day-21 lectures fix lives here, so that no other
+#  module contains a magic number. The turtle coordinate system has its centre
+#  at (0, 0); the 600 by 600 screen therefore reaches from -300 to 300 on both
+#  axes. A direction is a turtle heading in degrees: 0 points right and the
+#  angle grows counter-clockwise.
+
+"""Constants of the snake game, as fixed by the day-20 and day-21 lectures."""
+
+from typing import Final
+
+## @brief Width of the screen in pixels.
+SCREEN_WIDTH: Final[int] = 600
+
+## @brief Height of the screen in pixels.
+SCREEN_HEIGHT: Final[int] = 600
+
+## @brief Background colour of the screen.
+SCREEN_BACKGROUND_COLOR: Final[str] = "black"
+
+## @brief Title of the game window.
+SCREEN_TITLE: Final[str] = "My Snake Game"
+
+## @brief Shape of every segment of the snake.
+SEGMENT_SHAPE: Final[str] = "square"
+
+## @brief Colour of every segment of the snake.
+SEGMENT_COLOR: Final[str] = "white"
+
+## @brief Where the segments are drawn when the game starts, from head to tail.
+#
+#  The positions lie on one row, 20 pixels apart, which is the width of a
+#  default turtle square, so the segments touch without overlapping.
+STARTING_POSITIONS: Final[tuple[tuple[int, int], ...]] = ((0, 0), (-20, 0), (-40, 0))
+
+## @brief Distance in pixels that the head moves in one move.
+MOVE_DISTANCE: Final[int] = 20
+
+## @brief Direction up, as a turtle heading in degrees.
+UP: Final[int] = 90
+
+## @brief Direction down, as a turtle heading in degrees.
+DOWN: Final[int] = 270
+
+## @brief Direction left, as a turtle heading in degrees.
+LEFT: Final[int] = 180
+
+## @brief Direction right, as a turtle heading in degrees.
+RIGHT: Final[int] = 0
+
+## @brief Wait in seconds between two moves.
+REFRESH_DELAY_SECONDS: Final[float] = 0.1
+
+## @brief Shape of the food.
+FOOD_SHAPE: Final[str] = "circle"
+
+## @brief Stretch factor of the food in both directions; 0.5 is half a default turtle.
+FOOD_SIZE: Final[float] = 0.5
+
+## @brief Colour of the food.
+FOOD_COLOR: Final[str] = "blue"
+
+## @brief Speed of the food; `fastest` switches its animation off.
+FOOD_SPEED: Final[str] = "fastest"
+
+## @brief How far from the centre the wall is, on every side, in pixels.
+#
+#  The head must stay inside it, and the food is shown inside it.
+WALL_LIMIT: Final[int] = 280
+
+## @brief The snake eats the food when the head is closer to it than this, in pixels.
+FOOD_COLLISION_DISTANCE: Final[int] = 15
+
+## @brief Text in front of the score on the scoreboard.
+SCORE_LABEL: Final[str] = "Score: "
+
+## @brief Colour of the scoreboard text.
+SCOREBOARD_COLOR: Final[str] = "white"
+
+## @brief Where the scoreboard text is written: the top centre of the screen.
+SCOREBOARD_POSITION: Final[tuple[int, int]] = (0, 270)
+
+## @brief Alignment of the scoreboard text around its position.
+SCOREBOARD_ALIGNMENT: Final[str] = "center"
+
+## @brief Font of the scoreboard text: family, size and style.
+SCOREBOARD_FONT: Final[tuple[str, int, str]] = ("Arial", 24, "normal")
+
+## @brief The head touches the tail when it is closer to a segment than this, in pixels.
+TAIL_COLLISION_DISTANCE: Final[int] = 10
+
+## @brief Text shown when the game is over.
+GAME_OVER_TEXT: Final[str] = "GAME OVER"
+
+## @brief Where the game-over text is written: the centre of the screen.
+GAME_OVER_POSITION: Final[tuple[int, int]] = (0, 0)