fix: preserve gameplay data and correct duel evaluation

- Preserve snake customizations across database migrations and merges.
- Lazily load optional storage backends for SQLite maintenance scripts.
- Match Apex territory and nearest-food tie-breaking semantics.
- Resolve duel occupancy after simultaneous movement and food growth.
- Recompute simulated head-to-head danger after body growth.
- Add regression coverage and declare the aiofiles dependency.

snakes/bitboard_duel_search.py

@@ -129,8 +129,8 @@ class BitboardDuelSearch:
       if alpha >= beta:
         return cached_value, True
 
-    my_moves = self._legal_targets(state.my_body, state.enemy_body)
-    enemy_moves = self._legal_targets(state.enemy_body, state.my_body)
+    my_moves = self._candidate_targets(state.my_body)
+    enemy_moves = self._candidate_targets(state.enemy_body)
     if not my_moves:
       return self.LOSS - depth, True
     if not enemy_moves:
@@ -219,14 +219,13 @@ class BitboardDuelSearch:
     )
     return child, None
 
-  def _legal_targets(self, body: Body, other_body: Body) -> list[int]:
-    occupied = self._body_bits(body) | self._body_bits(other_body)
-    if not self._tail_stacked(body):
-      occupied &= ~(1 << body[-1])
-    if not self._tail_stacked(other_body):
-      occupied &= ~(1 << other_body[-1])
-    legal = self.board.neighbors_of(body[0]) & ~occupied & self.board.board_mask
-    return list(self._iter_bits(legal))
+  def _candidate_targets(self, body: Body) -> list[int]:
+    """Return in-bounds targets; `_advance` resolves simultaneous collisions.
+
+    Delaying occupancy checks until both targets and food growth are known is
+    essential: whether either tail vacates depends on that snake eating.
+    """
+    return list(self._iter_bits(self.board.neighbors_of(body[0])))
 
   def _ordered_moves(self, moves: list[int], state: DuelState, depth: int, mine: bool) -> list[int]:
     body = state.my_body if mine else state.enemy_body