Function subscribeDevtoolsEvent

Subscribe to the devtools event stream.

• Returns an UnsubscribeFn that removes this listener.
• Idempotent: subscribing the same function reference twice tracks it once (Set semantics).
• Throwing listeners do NOT block delivery to other listeners; the error is logged via console.warn and dispatch continues.
• Re-entrancy safe: listeners that subscribe/unsubscribe during dispatch affect future events only, not the in-flight one.