- Add WIN_SCORE, AI_EASY, AI_MEDIUM, AI_HARD to GameConfig
- Add paddle2, score1, score2, mode, difficulty, gameState, winner to GameState
- Replace anonymous Input handlers with named refs + cleanup() method
- Add Input.getVerticalInput2() for ArrowUp/ArrowDown keys
- Position paddle2 on right side in Physics.init()
- Update Physics.onResize() to call init() for full repositioning