- Add is_underwater() and set_underwater() to ClientPlayer.
- Change get_world() to return non-const reference for audio access.
- Update Camera::update_move_camera() to detect water block changes and trigger transition sound.
- Add repeating timer in ClientWorld to play random bubble ambient sounds while underwater.
- Include new ambient sound assets for water transitions and bubbles.
- Also add sand walk sound asset (unused in this commit).
