Design a BLE Command Handler

Design a Bluetooth Low Energy (BLE) command handler for an iOS application. The app needs to send commands to a wearable device over BLE. Commands must be processed strictly one at a time: - A command may not be sent until all earlier commands have completed. - After sending a command, the handler must wait for either: - the expected response from the device, or - a timeout. - If a command times out, the handler should unblock the queue and continue handling subsequent commands according to a clearly defined failure policy. - Multiple parts of the app may enqueue commands concurrently. - The design should be suitable for production use in a mobile app, including lifecycle, cancellation, error handling, and testability considerations. Explain the architecture, key data structures, concurrency model, timeout handling, response matching, and how you would validate the design.