* {@link NullPointerException}. This test is defensive; in real code it
     * would likely be handled elsewhere.
     */
    @Test
    @DisplayName("calling getters on null reference throws NPE")
    void testNullReference() {
        AllocInfo nullRef = null;
        assertThrows(NullPointerException.class, () -> nullRef.getCapacity());
        assertThrows(NullPointerException.class, () -> nullRef.getFree());
    }