Sync .ecx file before close matching Go's EcVolume.Close

Go calls ev.ecxFile.Sync() before closing to ensure in-place deletion
marks are flushed to disk. Without this, deletion marks written via
MarkNeedleDeleted could be lost on crash.

seaweed-volume/src/storage/erasure_coding/ec_volume.rs

@@ -749,6 +749,10 @@ impl EcVolume {
             }
             *shard = None;
         }
+        // Sync .ecx before closing to flush in-place deletion marks (matches Go's ev.ecxFile.Sync())
+        if let Some(ref ecx_file) = self.ecx_file {
+            let _ = ecx_file.sync_all();
+        }
         self.ecx_file = None;
         self.ecj_file = None;
     }