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/art.py

@@ -0,0 +1,31 @@
+"""@file art.py
+@brief ASCII art shown by the game, as given by the assignment.
+
+The lines are raw strings so the backslashes in the art survive unchanged.
+They are concatenated line by line, rather than written as one multi-line
+string, so Doxygen does not read the backticks in the art as markup.
+"""
+
+## The game logo.
+logo = (
+    "\n"
+    r"    __  ___       __             " "\n"
+    r"   / / / (_)___ _/ /_  ___  _____" "\n"
+    r"  / /_/ / / __ `/ __ \/ _ \/ ___/" "\n"
+    r" / __  / / /_/ / / / /  __/ /    " "\n"
+    r"/_/ ///_/\__, /_/ /_/\___/_/     " "\n"
+    r"   / /  /____/_      _____  _____" "\n"
+    r"  / /   / __ \ | /| / / _ \/ ___/" "\n"
+    r" / /___/ /_/ / |/ |/ /  __/ /    " "\n"
+    r"/_____/\____/|__/|__/\___/_/     " "\n"
+)
+
+## The "vs" shown between the two accounts.
+vs = (
+    "\n"
+    r" _    __    " "\n"
+    r"| |  / /____" "\n"
+    r"| | / / ___/" "\n"
+    r"| |/ (__  ) " "\n"
+    r"|___/____(_)" "\n"
+)