20260427_2 移除parallel phase开关，默认全量并行

src/parallel/config.rs

@@ -14,9 +14,7 @@ pub struct ParallelPhase2Config {
 impl Default for ParallelPhase2Config {
     fn default() -> Self {
         Self {
-            object_workers: std::thread::available_parallelism()
-                .map(|n| n.get().max(1))
-                .unwrap_or(4),
+            object_workers: 8,
             worker_queue_capacity: 256,
         }
     }