refactor: simplify game code

[milesapnash]

• Moved all classes into com.mapna.snake package
• Encapsulated Snake state: owns its own movement, protects body list with unmodifiable view, uses HashSet for O(1) collision checks
• Extracted game logic into testable GameEngine with injectable Random
• Added WON game state when snake fills the board
• Optimised food placement to enumerate free cells instead of retry loop
• Eliminated magic floats and duplicated direction/paint logic in BoardRenderer
• Extracted constants (HIGHSCORE_FILE, INITIAL_LENGTH, fonts) and removed excess final keywords
• Fixed rendering fragility: score overlay now sets its own font/color explicitly instead of relying on side-effects from paintTitles
• Removed unnecessary defensive copies on GameState.food
• Added SnakeTest (12 tests) and FileHighScoreStoreTest (9 tests)
• Added GitHub Actions CI pipeline (compile + test on push/PR)
• Added maven-shade-plugin for runnable JAR packaging