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/tests.py

@@ -12,7 +12,7 @@ in the folder where this file exists:
 """
 import unittest
 
-from snakes.LogicSnake import avoid_my_neck
+from snakes.legacy.LogicSnake import avoid_my_neck
 
 
 class AvoidNeckTest(unittest.TestCase):