Dynamic integer collection

Implement append_unique.

typedef struct {
    int *data;
    size_t size;
    size_t capacity;
} int_array_t;

int append_unique(
    int_array_t *array,
    int value
);

Add value only when it is not already present. Grow the allocation when size == capacity, preserve the existing allocation if realloc fails, and update size and capacity only after success. Return non-zero on success and 0 on allocation failure. Adding an existing value is successful but must not change the array.