[c-bata]

Summary

Study currently wraps trial queues in an outer RwLock. This change lets each queue implementation handle its own synchronization and allows the queue handle to be shared directly, following the approach used for samplers in #216.

• Change TrialQueue::enqueue and dequeue to take &self.
• Replace Arc<RwLock<dyn TrialQueue>> with Arc<dyn TrialQueue> in Study and the PyO3 bindings.
• Use a mutex for the in-memory queue and backend-level synchronization for directory and SQLite queues.
• Update the Rust and Python adapters to use the shared queue directly.

Compatibility

This changes the public Rust TrialQueue trait and the types of Study::new and Study::queue. Custom Rust implementations and callers that use the previous RwLock-wrapped queue will need to update.