feat(snake): add adaptive adversarial search

- Share duel search contexts and transpositions across candidate moves.
- Add aspiration windows, principal variation ordering, and body caches.
- Model simultaneous multiplayer responses with a compact beam rollout.
- Adapt search depth and response breadth to the remaining deadline.
- Add a deterministic arena benchmark with optional JSON reporting.
- Expose search metrics, document benchmarking, and bump Prism to 1.2.0.

snakes/bitboard.py

@@ -326,15 +326,15 @@ class BitBoard:
     # index when several foods are equally close, which can change contested-
     # food scoring and therefore the selected move.
     queue = [start_idx]
-    distances = [0]
     seen = start_bit
     cursor = 0
+    layer_end = 1
+    dist = 0
     w = self.width
     size = self.size
 
     while cursor < len(queue):
       cell = queue[cursor]
-      dist = distances[cursor]
       cursor += 1
       x = cell % w
       candidates = (
@@ -357,6 +357,8 @@ class BitBoard:
           return dist + 1, neighbor
         seen |= bit
         queue.append(neighbor)
-        distances.append(dist + 1)
+      if cursor == layer_end:
+        dist += 1
+        layer_end = len(queue)
 
     return None, None