Flush .ecx in EC volume sync_to_disk matching Go's Sync()

Go's EcVolume.Sync() flushes both the .ecj journal and the .ecx index
to disk. The Rust version only flushed .ecj, leaving in-place deletion
marks in .ecx unpersisted until close(). This could cause data
inconsistency if the server crashes after marking a needle deleted in
.ecx but before close().

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

@@ -178,11 +178,15 @@ impl EcVolume {
         format!("{}.ecj", self.idx_base_name())
     }
 
-    /// Sync the EC volume's journal file to disk (matching Go's ecv.SyncToDisk()).
+    /// Sync the EC volume's journal and index files to disk (matching Go's ecv.Sync()).
+    /// Go flushes both .ecj and .ecx to ensure in-place deletion marks are persisted.
     pub fn sync_to_disk(&self) -> io::Result<()> {
         if let Some(ref ecj_file) = self.ecj_file {
             ecj_file.sync_all()?;
         }
+        if let Some(ref ecx_file) = self.ecx_file {
+            ecx_file.sync_all()?;
+        }
         Ok(())
     }