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_run_seeded_snake_tournament.py

@@ -0,0 +1,54 @@
+import io
+import unittest
+from unittest.mock import patch
+
+from scripts.run_seeded_snake_tournament import ENGINE_USER_AGENT, run_game
+
+class _Completed:
+  returncode = 0
+  stdout = "INFO Game completed after 42 turns. Prism was the winner.\n"
+
+class _OutputFile:
+
+  def __init__(self, *args, **kwargs):
+    self.file = io.BytesIO(
+      b'{"turn":0}\n'
+      b'{"winnerId":"snake-id","winnerName":"Prism","isDraw":false}\n'
+    )
+    self.name = "arena-output.jsonl"
+
+  def __enter__(self):
+    return self
+
+  def __exit__(self, *args):
+    self.file.close()
+
+  def seek(self, offset):
+    return self.file.seek(offset)
+
+  def __iter__(self):
+    return iter(self.file)
+
+class TestSeededSnakeTournament(unittest.TestCase):
+
+  def test_proxy_identifies_requests_as_battlesnake_engine(self):
+    self.assertIn("BattlesnakeEngine", ENGINE_USER_AGENT)
+
+  @patch("scripts.run_seeded_snake_tournament.subprocess.run", return_value=_Completed())
+  @patch("scripts.run_seeded_snake_tournament.tempfile.NamedTemporaryFile", _OutputFile)
+  def test_run_game_reads_official_engine_result(self, run):
+    result = run_game(
+      cli="battlesnake", seed=7, game_type="standard", map_name="standard",
+      players=[("Apex", "http://host:9001"), ("Prism", "http://host:9002")],
+      width=11, height=11, timeout_ms=500,
+    )
+
+    self.assertEqual(result, {
+      "seed": 7, "winner": "Prism", "draw": False, "turns": 42,
+    })
+    command = run.call_args.args[0]
+    self.assertIn("--seed", command)
+    self.assertIn("--output", command)
+
+if __name__ == "__main__":
+  unittest.main()

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()