Function handle_append_block_file 

pub fn handle_append_block_file(
    broker: &Broker,
    block_id: &str,
    filename: &str,
    data: &[u8],
    filestore: Option<&Arc<FileStore>>,
)

and write-through to FileStore (if provided).

Port of Go’s HandleAppendBlockFile.

The FileStore write is fire-and-forget: if it fails we emit a warning but never propagate the error back to the hot stdout-reader path.