feat: add gameplay stats and restore Zenith rollout depth

- Add read-only gameplay statistics with filters and opponent comparisons.
- Report move-trace latency percentiles and external timeout counts.
- Pair engine latency with the preceding turn to separate timing stages.
- Adjust Zenith rollout deepening for its larger timeout reserve.
- Bump Zenith to 1.2.0 and document the fix and snake-stats command.
- Cover statistics, trace aggregation, rollout depth, and verdict budgets.

snakes/engine/survival.py

@@ -4,6 +4,10 @@ from time import perf_counter
 from snakes.engine.survival_search import CompactSurvivalSearch
 
 class BitboardSurvivalMixin:
+  # Remaining budget above which rollouts against at most two enemies may look
+  # one turn deeper. Snakes with a larger timeout reserve lower this threshold.
+  _rollout_deepen_ms = 250.0
+
   def _future_rollout_bonus(
       self, move: str, safe_moves: dict, my_body: list, other_snakes: list,
       food_set: set, is_constrictor: bool, width: int, height: int,
@@ -36,7 +40,7 @@ class BitboardSurvivalMixin:
       depth = min(self._planning_depth, 2 if len(other_snakes) > 1 else 3)
       if remaining < 90:
         depth = min(depth, 2)
-      elif remaining > 250 and len(other_snakes) <= 2:
+      elif remaining > self._rollout_deepen_ms and len(other_snakes) <= 2:
         depth = min(4, depth + 1)
       raw = self._survival_search_context.search_selected(
         my_body=my_body,