MOAD-0003 (Leaked Context): CLEAN

darktable uses `static __thread int32_t threadid` in control/jobs.c to track
numeric worker thread pool indices. This is not request-scoped identity: it is
a pool slot number used for selecting per-thread OpenCL queues and cache
entries. It carries no user-visible identity (image ID, job ID, session). The
correct fix pattern (ScopedValue / ContextVar) does not apply here. No Leaked
Context defect found.
