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/test_MasterSnake.py

@@ -1,5 +1,5 @@
 import unittest
-from snakes.MasterSnake import MasterSnake
+from snakes.legacy.MasterSnake import MasterSnake
 
 class TestMasterSnake(unittest.TestCase):
     def setUp(self):