MOAD-0005 (Thundering Herd / CWE-362): CLEAN

darktable's image cache (src/common/cache.c) uses a single dt_pthread_mutex_t
covering all get/insert/release operations. The dt_cache_get function acquires
the lock before hash lookup and holds it through the entire miss path. No
concurrent get+null+compute+put without synchronization pattern found.
The mipmap/thumbnail cache follows the same pattern. No Thundering Herd
defect found.
