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/snakes/test_SnakePackageLayout.py

@@ -0,0 +1,35 @@
+import unittest
+
+from snakes import SNAKE_REGISTRATIONS, SnakeBuilder
+from snakes.engine import (
+  BitBoard,
+  BitboardDuelMixin,
+  BitboardSpatialMixin,
+  BitboardSurvivalMixin,
+)
+from snakes.strategies.prism import PrismBattleSnake_GPT_5_6_Sol
+
+class TestSnakePackageLayout(unittest.TestCase):
+
+  def test_registry_uses_explicit_package_modules(self):
+    for name, registration in SNAKE_REGISTRATIONS.items():
+      with self.subTest(name=name):
+        self.assertTrue(registration.module.startswith("snakes."))
+        self.assertNotEqual(registration.module, f"snakes.{name}")
+
+  def test_registry_builds_active_strategies_after_package_move(self):
+    for name in ("ApexBattleSnake", "PrismBattleSnake_GPT_5_6_Sol"):
+      with self.subTest(name=name):
+        snake = SnakeBuilder.build(name)
+        self.assertEqual(snake.__class__.__name__, name)
+
+  def test_prism_composes_reusable_engine_mixins(self):
+    snake = PrismBattleSnake_GPT_5_6_Sol()
+
+    self.assertIsInstance(snake, BitboardDuelMixin)
+    self.assertIsInstance(snake, BitboardSpatialMixin)
+    self.assertIsInstance(snake, BitboardSurvivalMixin)
+    self.assertIsInstance(snake._get_bb(11, 11), BitBoard)
+
+if __name__ == "__main__":
+  unittest.main()