PR #37 inlined VadWorkerPolicy into the desktop capture path but missed
the iOS files. Instead of restoring the deleted types, inline the same
direct if-let-Some pattern into ios_voice_unit.rs (the only remaining
iOS backend) and permanently remove VadWorkerPolicy and
callback_vad_worker_policy from vad/mod.rs.
iOS uses Option<AppleCoreMlVadWorker> with &mut self (no Mutex), so the
inline is simpler than the desktop's try_lock pattern.
Edison Jwa
413f247378