feat: port js13k 'The Power of Two' as fullscreen plugin

- Removed prime_snake demo game
- Added power_of_two: classic js13k math puzzle by Xem (2024)
- Calculator UI with Canvas rendering in fullscreen panel
- 17 tutorial levels + 1000 infinite levels (1..1000 with ≤8 twos)
- Uses IJS.io for progress saving, IJS.input for touch
- Digit shortcuts (levels > 27) for faster input
- Level completion melodies via IJS.audio

Co-Authored-By: Claude <noreply@anthropic.com>