Add the console game loop and entry point

Add show_round, ask_choice and play_game with injectable input and
output, and make python -m higher_lower play a game. Each account is
shown once per game; a wrong guess or running out of accounts ends it.
Invalid input is re-asked. Ctrl+C and Ctrl+D leave with a goodbye.
Cover the loop with scripted tests (36 passing).

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5
Closes #12
Closes #13
Closes #14
Closes #15
Closes #16

src/higher_lower/__main__.py

@@ -2,12 +2,17 @@
 @brief Entry point for `python -m higher_lower`.
 """
 
+from higher_lower.constants import MSG_GOODBYE
+from higher_lower.game import play_game
+
 
 def main() -> None:
-    """@brief Start the game.
-
-    Placeholder: the game loop is implemented in MIL-003.
-    """
+    """@brief Play a game; leave quietly if the player quits with Ctrl+C or Ctrl+D."""
+    try:
+        play_game()
+    except (EOFError, KeyboardInterrupt):
+        print()
+        print(MSG_GOODBYE)
 
 
 if __name__ == "__main__":

src/higher_lower/constants.py

@@ -26,3 +26,4 @@ MSG_INVALID_CHOICE = "Please type 'A' or 'B'."
 MSG_CORRECT = "You're right! Current score: {score}."
 MSG_WRONG = "Sorry, that's wrong. Final score: {score}"
 MSG_WIN = "You went through every account! Final score: {score}"
+MSG_GOODBYE = "Goodbye!"

src/higher_lower/game.py

@@ -0,0 +1,122 @@
+"""@file game.py
+@brief The interactive game: what the player sees, what they type, and the game loop.
+
+Input and output go through callables so tests can script a whole game.
+"""
+
+import random
+from collections.abc import Callable, Sequence
+
+from higher_lower.art import logo, vs
+from higher_lower.constants import (
+    CHOICE_A,
+    CHOICE_B,
+    KEY_FOLLOWER_COUNT,
+    KEY_NAME,
+    MSG_AGAINST_B,
+    MSG_COMPARE_A,
+    MSG_CORRECT,
+    MSG_INVALID_CHOICE,
+    MSG_WIN,
+    MSG_WRONG,
+    PROMPT_GUESS,
+    START_SCORE,
+)
+from higher_lower.game_data import Account, data
+from higher_lower.game_logic import (
+    check_answer,
+    format_data,
+    get_random_account,
+    pick_pair,
+)
+
+InputFunc = Callable[[str], str]
+OutputFunc = Callable[[str], None]
+
+
+def show_round(
+    account_a: Account,
+    account_b: Account,
+    output: OutputFunc = print,
+) -> None:
+    """@brief Show the two accounts the player compares.
+
+    @param account_a The account shown as A.
+    @param account_b The account shown as B.
+    @param output Where the text goes.
+    """
+    output(MSG_COMPARE_A.format(account=format_data(account_a)))
+    output(vs)
+    output(MSG_AGAINST_B.format(account=format_data(account_b)))
+
+
+def ask_choice(input_func: InputFunc = input, output: OutputFunc = print) -> str:
+    """@brief Ask for A or B until the player gives a valid answer.
+
+    @param input_func Reads one line of input, given the prompt.
+    @param output Where the invalid-answer message goes.
+    @return "a" or "b", in lower case.
+    """
+    while True:
+        answer = input_func(PROMPT_GUESS).strip().lower()
+        if answer in (CHOICE_A, CHOICE_B):
+            return answer
+        output(MSG_INVALID_CHOICE)
+
+
+def _pick_next_account(
+    accounts: Sequence[Account],
+    shown_names: set[str],
+    rng: random.Random | None,
+) -> Account | None:
+    """@brief Draw an account that has not been shown in this game.
+
+    @param accounts All accounts of the game.
+    @param shown_names Names of the accounts already shown.
+    @param rng Random generator, or None for the default.
+    @return A new account, or None if every account has been shown.
+    """
+    unused = [account for account in accounts if account[KEY_NAME] not in shown_names]
+    if not unused:
+        return None
+    return get_random_account(unused, rng)
+
+
+def play_game(
+    accounts: Sequence[Account] = data,
+    rng: random.Random | None = None,
+    input_func: InputFunc = input,
+    output: OutputFunc = print,
+) -> int:
+    """@brief Play one game, from the logo to the final score.
+
+    Each account is shown once per game. A right guess adds a point and
+    makes B the next A; a wrong guess, or running out of accounts, ends
+    the game.
+
+    @param accounts The accounts to play with; needs at least two entries.
+    @param rng Random generator; pass a seeded one for a repeatable game.
+    @param input_func Reads one line of input, given the prompt.
+    @param output Where the text goes.
+    @return The final score.
+    """
+    output(logo)
+    score = START_SCORE
+    account_a, account_b = pick_pair(accounts, rng)
+    shown_names = {account_a[KEY_NAME], account_b[KEY_NAME]}
+    while True:
+        show_round(account_a, account_b, output)
+        guess = ask_choice(input_func, output)
+        if not check_answer(
+            guess, account_a[KEY_FOLLOWER_COUNT], account_b[KEY_FOLLOWER_COUNT]
+        ):
+            output(MSG_WRONG.format(score=score))
+            return score
+        score += 1
+        output(MSG_CORRECT.format(score=score))
+        next_account = _pick_next_account(accounts, shown_names, rng)
+        if next_account is None:
+            output(MSG_WIN.format(score=score))
+            return score
+        account_a, account_b = account_b, next_account
+        shown_names.add(next_account[KEY_NAME])