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.

tests/bench_best_battle_snake.py

@@ -3,7 +3,7 @@ import argparse
 import time
 
 from server.GameBoard import GameBoard
-from snakes.BestBattleSnake import BestBattleSnake
+from snakes.legacy.BestBattleSnake import BestBattleSnake
 
 def build_game_state() -> dict:
   return {