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_PrismBattleSnake_GPT_5_6_Sol.py

@@ -2,11 +2,11 @@ import unittest
 from time import perf_counter
 
 from snakes import SnakeBuilder, get_snake_version
-from snakes.ApexBattleSnake import ApexBattleSnake
-from snakes.bitboard import BitBoard
-from snakes.bitboard_duel_search import BitboardDuelSearch
-from snakes.compact_survival_search import CompactSurvivalSearch
-from snakes.PrismBattleSnake_GPT_5_6_Sol import PrismBattleSnake_GPT_5_6_Sol
+from snakes.engine.bitboard import BitBoard
+from snakes.engine.duel_search import BitboardDuelSearch
+from snakes.engine.survival_search import CompactSurvivalSearch
+from snakes.strategies.apex import ApexBattleSnake
+from snakes.strategies.prism import PrismBattleSnake_GPT_5_6_Sol
 
 class TestBitBoard(unittest.TestCase):
 
@@ -52,8 +52,9 @@ class TestPrismBattleSnake_GPT_5_6_Sol(unittest.TestCase):
     snake = PrismBattleSnake_GPT_5_6_Sol()
 
     self.assertEqual(snake.name, "PrismBattleSnake")
-    self.assertEqual(snake.version, "1.2.0")
-    self.assertEqual(get_snake_version("PrismBattleSnake_GPT_5_6_Sol"), "1.2.0")
+    self.assertEqual(snake.version, "1.3.0")
+    self.assertEqual(get_snake_version("PrismBattleSnake_GPT_5_6_Sol"), "1.3.0")
+    self.assertGreaterEqual(snake._planning_depth, 4)
     self.assertIsInstance(SnakeBuilder.build("PrismBattleSnake_GPT_5_6_Sol"), PrismBattleSnake_GPT_5_6_Sol)
 
   def test_bitboard_primitives_match_apex(self):
@@ -175,6 +176,21 @@ class TestPrismBattleSnake_GPT_5_6_Sol(unittest.TestCase):
 
     self.assertIs(snake._duel_search_context, first_context)
     self.assertGreater(first_context.nodes, 0)
+    self.assertGreater(first_context.completed_depth, 0)
+
+  def test_duel_evaluation_prioritizes_reachable_food_when_starving(self):
+    board = BitBoard(5, 3)
+    search = BitboardDuelSearch(
+      board=board, food={(2, 1)}, hazards=set(), hazard_count={},
+      hazard_damage=15, deadline=None,
+    )
+    my_body = [{"x": 0, "y": 1}, {"x": 0, "y": 0}]
+    enemy_body = [{"x": 4, "y": 1}, {"x": 4, "y": 0}]
+
+    hungry = search.search_depth(my_body, enemy_body, 15, 100, 0, set())
+    healthy = search.search_depth(my_body, enemy_body, 100, 100, 0, set())
+
+    self.assertLess(hungry, healthy)
 
   def test_compact_rollout_models_lethal_enemy_head_response(self):
     board = BitBoard(3, 3)
@@ -205,6 +221,7 @@ class TestPrismBattleSnake_GPT_5_6_Sol(unittest.TestCase):
     search.search_selected(mine, enemies, (2, 1), depth=3)
 
     self.assertGreater(search.cache_hits, hits_before)
+    self.assertGreaterEqual(search.completed_depth, 3)
 
   def test_bitboard_duel_search_reuses_transpositions(self):
     board = BitBoard(5, 5)

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()

tests/snakes/test_SupremeBattleSnake.py

@@ -5,8 +5,8 @@ and that the bitboard engine itself is sound.
 """
 import unittest
 
-from snakes.SupremeBattleSnake_ClaudeOpus4_6 import SupremeBattleSnake_ClaudeOpus4_6 as SupremeBattleSnake
-from snakes.bitboard import BitBoard
+from snakes.legacy.SupremeBattleSnake_ClaudeOpus4_6 import SupremeBattleSnake_ClaudeOpus4_6 as SupremeBattleSnake
+from snakes.engine.bitboard import BitBoard
 from server.GameBoard import GameBoard
 
 # ── Helpers ───────────────────────────────────────────────────────────────────
@@ -348,7 +348,7 @@ class TestParityWithApex(unittest.TestCase):
 
   def test_trapped_corner(self):
     """Both snakes should survive a forced single-exit scenario."""
-    from snakes.ApexBattleSnake import ApexBattleSnake
+    from snakes.strategies.apex import ApexBattleSnake
 
     state = gs(my_body=[(1, 1), (1, 2), (2, 2), (2, 1)],
                other_bodies=[], foods=[(5, 5)], width=7, height=7)

tests/snakes/test_UltimateBattleSnake.py

@@ -1,6 +1,6 @@
 import unittest
 
-from snakes.UltimateBattleSnake import UltimateBattleSnake
+from snakes.legacy.UltimateBattleSnake import UltimateBattleSnake
 from server.GameBoard import GameBoard
 
 # ── Helpers ───────────────────────────────────────────────────────────────────