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/scripts/test_snake_arena_scenarios.py

@@ -0,0 +1,31 @@
+import unittest
+
+from scripts.snake_arena_scenarios import SCENARIOS, synthetic_states
+
+class TestSnakeArenaScenarios(unittest.TestCase):
+
+  def test_default_corpus_rotates_through_every_scenario(self):
+    states = synthetic_states(len(SCENARIOS))
+
+    self.assertEqual(
+      {metadata["scenario"] for _, metadata in states},
+      set(SCENARIOS),
+    )
+
+  def test_scenario_filter_is_deterministic(self):
+    first = synthetic_states(3, ["hazard"])
+    second = synthetic_states(3, ["hazard"])
+
+    self.assertEqual(first, second)
+    self.assertTrue(all(metadata["scenario"] == "hazard" for _, metadata in first))
+
+  def test_generated_you_is_present_on_board(self):
+    for board, metadata in synthetic_states(20):
+      with self.subTest(scenario=metadata["scenario"]):
+        ids = {snake["id"] for snake in board["snakes"]}
+        self.assertIn(metadata["you"]["id"], ids)
+        self.assertGreater(board["width"], 0)
+        self.assertGreater(board["height"], 0)
+
+if __name__ == "__main__":
+  unittest.main()