feat(core): add wakeup callback for external event loop integration

[rlch]

Summary

Adds VirtualDom::set_wakeup_callback() — a hook that fires when async tasks complete, allowing external event loops to schedule a repaint without polling.

The problem: Frameworks that drive the VirtualDom synchronously (calling render_immediate() in response to input events) have no way to know when async work needs processing. A use_resource fetch that completes while the user isn't interacting sits in the VirtualDom's queue until the next input event. This causes visible delays — e.g., images only appearing when the user hovers over the UI.

The solution: set_wakeup_callback registers a Send + Sync callback on the runtime that gets invoked from ArcWake::wake_by_ref alongside the existing channel send. External event loops use this to schedule a render_immediate call:

// winit example
let proxy = event_loop.create_proxy();
dom.set_wakeup_callback(move || {
    let _ = proxy.send_event(UserEvent::NewWork);
});

This follows the universal pattern used by every major GUI framework (Flutter's scheduleFrame, egui's request_repaint, Iced's EventLoopProxy, Qt's postEvent, GTK's g_idle_add).

Changes:

• Runtime: new wakeup_callback: RefCell<Option<Arc<dyn Fn() + Send + Sync>>> field
• LocalTaskHandle: carries the callback, invokes it in wake_by_ref
• VirtualDom::set_wakeup_callback(): public API to set the callback
• 2 new tests in packages/core/tests/wakeup_callback.rs

Test plan

• All existing dioxus-core tests pass (71 passed, 7 ignored)
• New test: callback fires when spawned async task completes
• New test: callback does NOT fire when no async work is spawned
• No breaking changes — fully backwards compatible (callback is optional, defaults to None)

[ealmloff]

The wait_for_work method on the virtual dom will poll any futures that have woken up and return when a scope needs to be rerun. We use this with a custom waker to integrate with the window event loop here in desktop:

dioxus/packages/desktop/src/webview.rs

Line 539 in 45442af

pub fn poll_vdom(&mut self) {