Add game data, art, constants and game logic

Add the assignment data set (50 accounts), the logo and vs art, the
constants module, and the pure functions get_random_account,
pick_pair, format_data and check_answer, all with Doxygen comments.
pick_pair never returns the same account twice; check_answer accepts
either choice on a tie. Cover them with pytest tests (22 passing).

The art is built from concatenated raw lines so the backslashes survive
and Doxygen does not read the backticks as markup.

Accept MIL-002 after review RC-005 and add it to the Traceability
Matrix.

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Closes #6
Closes #7
Closes #8
Closes #9
Closes #10
Closes #11

src/higher_lower/game_logic.py

@@ -0,0 +1,78 @@
+"""@file game_logic.py
+@brief Pure functions of the game: drawing accounts, describing them and checking guesses.
+"""
+
+import random
+from collections.abc import Sequence
+
+from higher_lower.constants import (
+    ACCOUNT_TEMPLATE,
+    CHOICE_A,
+    CHOICE_B,
+    KEY_DESCRIPTION,
+    KEY_NAME,
+    KEY_COUNTRY,
+)
+from higher_lower.game_data import Account, data
+
+_DEFAULT_RNG = random.Random()
+
+
+def get_random_account(
+    accounts: Sequence[Account] = data,
+    rng: random.Random | None = None,
+) -> Account:
+    """@brief Return one random account.
+
+    @param accounts The accounts to draw from.
+    @param rng Random generator; pass a seeded one to make the draw repeatable.
+    @return A random entry of accounts.
+    """
+    return (rng or _DEFAULT_RNG).choice(accounts)
+
+
+def pick_pair(
+    accounts: Sequence[Account] = data,
+    rng: random.Random | None = None,
+) -> tuple[Account, Account]:
+    """@brief Return two different random accounts.
+
+    @param accounts The accounts to draw from; needs at least two entries.
+    @param rng Random generator; pass a seeded one to make the draw repeatable.
+    @return The accounts A and B, never the same entry.
+    @throws ValueError If accounts has fewer than two entries.
+    """
+    first, second = (rng or _DEFAULT_RNG).sample(accounts, 2)
+    return first, second
+
+
+def format_data(account: Account) -> str:
+    """@brief Describe an account for the player.
+
+    @param account The account to describe.
+    @return Text such as "Nike, a Sportswear multinational, from United States".
+    """
+    return ACCOUNT_TEMPLATE.format(
+        name=account[KEY_NAME],
+        description=account[KEY_DESCRIPTION],
+        country=account[KEY_COUNTRY],
+    )
+
+
+def check_answer(guess: str, a_followers: int, b_followers: int) -> bool:
+    """@brief Tell whether the guess names the account with more followers.
+
+    Equal follower counts make either choice correct.
+
+    @param guess "a" or "b", in any case.
+    @param a_followers Follower count of account A.
+    @param b_followers Follower count of account B.
+    @return True if the guess is right.
+    @throws ValueError If guess is neither "a" nor "b".
+    """
+    choice = guess.lower()
+    if choice == CHOICE_A:
+        return a_followers >= b_followers
+    if choice == CHOICE_B:
+        return b_followers >= a_followers
+    raise ValueError(f"guess must be {CHOICE_A!r} or {CHOICE_B!r}, got {guess!r}")