feat(math-sprint): add game manifest

Add game.yaml with metadata for Math Sprint:
- Display name, icon, description
- Max 6 players
- Difficulty: Beginner
- Tags: Multiplayer, Free-for-All, Math Skills, Speed
- Purple color theme to match UI
- Set available: true

This manifest enables Math Sprint to appear in GameSelector
automatically via the registry system.

apps/web/src/arcade-games/math-sprint/game.yaml

@@ -0,0 +1,16 @@
+name: math-sprint
+displayName: Math Sprint
+icon: 🧮
+description: Fast-paced math racing game
+longDescription: Race against other players to solve math problems! Answer questions quickly to earn points. First person to answer correctly wins the round. Features multiple difficulty levels and customizable question counts.
+maxPlayers: 6
+difficulty: Beginner
+chips:
+  - 👥 Multiplayer
+  - ⚡ Free-for-All
+  - 🧮 Math Skills
+  - 🏃 Speed
+color: purple
+gradient: linear-gradient(135deg, #ddd6fe, #c4b5fd)
+borderColor: purple.200
+available: true