CoreBluetooth: concurrent connect/disconnect/discover drop the earlier caller's future

[qdot]

src/corebluetooth/internal.rs connect_peripheral (~L1117), disconnect_peripheral (~L1130), and discover_services (~L1497) store the reply future in a single Option slot (connected_future_state, disconnected_future_state, services_discovered_future_state). A second concurrent call overwrites the slot, so the first caller's future is dropped without a reply and its .await hangs forever.

Failure scenario: two tasks call peripheral.connect() concurrently; one of them never returns.

Fix direction: either queue waiters (e.g. Vec/VecDeque and resolve all) or immediately error the superseded future.