feat: ludo — 60fps render loop with offscreen board cache + smooth tween animations

- Offscreen canvas caches static board (drawn once, reused every frame)
- Continuous requestAnimationFrame loop renders pieces at 60fps
- Piece movement uses smooth parabolic arc tweens (no setTimeout stepping)
- Enter-from-home uses easeOutBounce for natural pop-in feel
- All drawBoard() calls now just mark board dirty — next frame picks it up
- Move preview ghosts pulse smoothly using sin wave
- Ambient corner glow rendered per-frame (lightweight, no cache needed)
- Render loop auto-stops on game exit/end for zero resource leak
- Bot step animation also uses smooth tween system
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>