Notifier channel inaccessible to enclave due to `mpsc` constraint

## Summary

The notifier channel (which rn only sends notifications when a handshake completes) is implemented using an `mpsc` channel, meaning it supports only a single consumer. By design, that single receiver is already used up by the host implementation (e.g., `DefaultHost`), so any enclave implementation built on top of Quartz can’t make another receive call.

In practice, though, the behavior is closer to a single producer, multiple consumers. So switching to a `broadcast` channel would make more sense here.

Locate the notifier channel and the DefaultHost implementation, then trace how handshake-completion notifications are sent and received. Check whether multiple enclave consumers are supported and identify the relevant tests or call sites; done means the host still receives notifications while an enclave implementation can receive them too.

Written by the indexing model from the issue text.