feat: Add Galaga arcade game with tutorial

Implement complete Galaga clone using ClojureScript and Scittle with
comprehensive tutorial documentation.

Features:
- Formation-based enemy AI (bee, butterfly, boss types)
- Swooping attack patterns with path-following
- Web Audio API sound effects (laser, explosion, swoop, victory)
- Mobile touch controls + desktop keyboard controls
- Particle effects, scrolling starfield, wave progression
- Score tracking with high scores and lives system

Technical:
- Reagent atom state management
- Canvas rendering with emoji sprites
- Enemy state machine (formation → swooping → returning)
- 60 FPS game loop with collision detection
- Keyword arguments for readable code

Documentation:
- 843-line tutorial with code examples and Mermaid diagrams
- Explains formation patterns, AI, audio system, game loop
- Clay/Quarto blog metadata for publishing

Files: galaga.clj (tutorial), galaga.cljs (game), 3 PNG screenshots

Author: burinc