Build an Opaque Cache

Implement a tiny fixed-size cache behind an opaque API: cache_t *cache_create(size_t cap), int cache_put(cache_t *c, int key, int value) (returns 1 stored, 0 full), int *cache_get(cache_t *c, int key) (returns NULL on miss). Keep the struct definition in your source; the tests only use the functions — that is the opacity contract.