Feature request: thread-safe wake handle for external event sources

Problem

Applications may receive events from long-lived external sources such as child processes, sockets, file watchers, or application-managed worker threads.

Currently, there does not appear to be a public way for such a thread to notify the UI event loop and produce a Msg. Applications must periodically poll a shared queue with a timer, even when no data is available.

This causes unnecessary updates/rebuilds and can interfere with animations such as caret blinking.

Proposed API

Please consider exposing a lightweight, thread-safe external wake handle:

const wake = try fx.externalWake(
    Effects.wakeMsg(.transport_ready),
);

// Called from an application-managed background thread.
try wake.notify();

References

Equivalent to winit's EventLoopProxy or libuv's uv_async_t

Start by tracing the event-loop integration and the existing Effects API around the proposed Effects.wakeMsg(.transport_ready) call. Compare the requested behavior with winit's EventLoopProxy and libuv's uv_async_t references. Done should provide a public externalWake handle whose notify() can be called from an application-managed background thread without timer polling.