CoreBluetooth: reachable panics in service discovery kill the CB thread

[qdot]

Two reachable panics in CoreBluetooth discovery run on the CB event-loop thread. When they fire, the thread dies and every subsequent/in-flight operation hangs or fails with "Channel closed".

1. src/corebluetooth/internal.rs check_discovered (~L345): panic!("We should still have a future at this point!"). set_characteristic_descriptors calls check_discovered whenever all characteristics are discovered; after the first discovery completes services_discovered_future_state is None, so any late descriptor callback (re-discovery, included service that is also primary, see Race condition causes panic on subscription in CoreBluetooth #167) panics.
2. internal.rs set_characteristics (~L276): .expect("Got characteristics for a service we don't know about"). central_delegate.rs requests characteristics for included services, which aren't in the service map from discoverServices(nil); also reachable after on_services_modified clears services while characteristic callbacks are in flight.

Fix direction: treat both as benign (trace-log and ignore / no-op when there is no waiting future or unknown service).