w.join(2000);
        assertFalse(w.isAlive(), "WriterThread should stop after -1 sentinel");
    }

    @Test
    @DisplayName("WriterThread surfaces SmbException via checkException")
    void writerThread_propagates_exception() throws Exception {
        // Arrange
        WriterThread w = new WriterThread();
        SmbFileOutputStream out = mock(SmbFileOutputStream.class);
        byte[] payload = new byte[] { 9, 8, 7 };