feat(snake): add adaptive adversarial search

- Share duel search contexts and transpositions across candidate moves.
- Add aspiration windows, principal variation ordering, and body caches.
- Model simultaneous multiplayer responses with a compact beam rollout.
- Adapt search depth and response breadth to the remaining deadline.
- Add a deterministic arena benchmark with optional JSON reporting.
- Expose search metrics, document benchmarking, and bump Prism to 1.2.0.

snakes/__init__.py

@@ -11,7 +11,7 @@ SNAKE_REGISTRY = {
   "UltimateBattleSnake": "4.5.0",
   "ApexBattleSnake": "1.0.0",
   "SupremeBattleSnake_ClaudeOpus4_6": "1.0.0",
-  "PrismBattleSnake_GPT_5_6_Sol": "1.1.0",
+  "PrismBattleSnake_GPT_5_6_Sol": "1.2.0",
 }
 
 DEFAULT_SNAKE_CONFIG = {