feat(snake): modularize engine and add tournament tools

- Split active strategies, reusable engine code, core classes, and legacy snakes.
- Replace implicit snake imports with explicit module registrations.
- Extract Prism duel, spatial, and survival behavior into focused mixins.
- Improve duel scoring with food races, pressure, caches, and depth metrics.
- Add deterministic arena scenarios and paired seeded engine tournaments.
- Expand benchmark telemetry and bump Prism to version 1.3.0.
- Update documentation and tests for the new package layout and tooling.

snakes/strategies/__init__.py

@@ -0,0 +1,6 @@
+"""Actively maintained competitive snake strategies."""
+
+from snakes.strategies.apex import ApexBattleSnake
+from snakes.strategies.prism import PrismBattleSnake_GPT_5_6_Sol
+
+__all__ = ("ApexBattleSnake", "PrismBattleSnake_GPT_5_6_Sol")