fix(core,protocol): simplify store_protocol and add download size cap

- store_protocol: always write to shared Arc<Mutex<Option<ProtocolClient>>>;
  the FileTransferService holds the same Arc so it sees updates automatically
- read_download_bytes: reject downloads exceeding 10 MB to prevent
  malicious servers from causing OOM

crates/chanora_protocol/src/adapter.rs

@@ -1093,7 +1093,15 @@ fn handle_download_failure(
     }
 }
 
+const MAX_DOWNLOAD_SIZE: u64 = 10 * 1024 * 1024;
+
 async fn read_download_bytes(result: FileDownloadResult) -> Result<Vec<u8>, ProtocolError> {
+    if result.size > MAX_DOWNLOAD_SIZE {
+        return Err(ProtocolError::FileTransfer(format!(
+            "download too large: {} bytes (max {})",
+            result.size, MAX_DOWNLOAD_SIZE
+        )));
+    }
     let size = usize::try_from(result.size).map_err(|_| {
         ProtocolError::FileTransfer(format!("download too large to buffer: {} bytes", result.size))
     })?;