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

tests/test_game_data.py

@@ -0,0 +1,46 @@
+"""@file test_game_data.py
+@brief Tests for the assignment data set and art.
+"""
+
+from higher_lower.art import logo, vs
+from higher_lower.constants import (
+    KEY_COUNTRY,
+    KEY_DESCRIPTION,
+    KEY_FOLLOWER_COUNT,
+    KEY_NAME,
+)
+from higher_lower.game_data import data
+
+EXPECTED_KEYS = {KEY_NAME, KEY_FOLLOWER_COUNT, KEY_DESCRIPTION, KEY_COUNTRY}
+
+
+def test_data_has_fifty_accounts() -> None:
+    """@brief The assignment data set has 50 entries."""
+    assert len(data) == 50
+
+
+def test_every_account_has_the_required_keys() -> None:
+    """@brief Every entry has exactly the four assignment keys."""
+    assert all(set(account) == EXPECTED_KEYS for account in data)
+
+
+def test_follower_counts_are_positive_integers() -> None:
+    """@brief Follower counts are positive integers."""
+    assert all(
+        isinstance(account[KEY_FOLLOWER_COUNT], int) and account[KEY_FOLLOWER_COUNT] > 0
+        for account in data
+    )
+
+
+def test_account_names_are_unique() -> None:
+    """@brief No account appears twice."""
+    names = [account[KEY_NAME] for account in data]
+    assert len(names) == len(set(names))
+
+
+def test_art_is_available() -> None:
+    """@brief The logo and the vs art keep their shape, including backslashes."""
+    assert len(logo.splitlines()) == 10
+    assert "\\" in logo
+    assert len(vs.splitlines()) == 6
+    assert "|___/____(_)" in vs