Function test_iterator_arithmetic

tests/test_container_helpers.h:732–743

// Test iterator arithmetic for RandomAccessIterator
template<typename Container>
void test_iterator_arithmetic() {
    Container c;
    populate(c, make_shared_int(10));
    populate(c, make_shared_int(20));
    populate(c, make_shared_int(30));

    // Test operator+ on iterators - COMPILER ERROR if operator+ doesn't exist
    auto it0 = c.begin();
    auto it1 = c.begin() + 1;
    FL_CHECK(it1 - it0 == 1);  // Distance check
    FL_CHECK(*(c.begin() + 1) != *c.begin());  // Different elements
}

// Test container comparison operators
template<typename Container>