A queued Envelope::Call was stranded until the last Sender dropped, so a
caller parked in gen_server::call was never released with ServerDown when a
*named* server was request_stop'd — the registry's inbox Sender clone (lazy
prune) kept the channel Arc, and the queued reply_tx, alive indefinitely.
Receiver::Drop now drains the queue (items dropped after releasing the lock,
since a reply_tx drop reaches a different channel's lock + the scheduler),
restoring the documented ServerDown guarantee on every teardown path.
Adds tests/stop_with_queued_call.rs: deterministic pure-smarm reproducer.
