OOP refactor: state machine, Level, and Audio

[Ansel7]

Restores the deleted Game.cpp and rewrites the project as a real object-oriented loop.

What changed

• Game is now a five-state machine: MainMenu, Instructions, Playing, Paused, GameOver.
• Each frame is handleInput() → update() → render() instead of one 500-line run().
• New Level class owns BSP load, FPS camera, collision, and chair picking.
• New Audio class owns a single IrrKlang engine (the old code created two).
• HUD widgets are created once and updated, not rebuilt every frame.
• Shared Ids.h for pick flags and GUI button IDs.
• Game.vcxproj lists Audio.cpp and Level.cpp.

Gameplay and assets are unchanged: find the chair 10 times before the 30-second timer hits zero.

Review notes are in REVIEW.md.