feat: add WindowIdExt::with_window_handle for native window handle access

[zendy00]

What

Adds WindowIdExt::with_window_handle, a backend-neutral way to obtain a window's native handle from a WindowId:

fn with_window_handle<F, T>(&self, f: F) -> Option<T>
where
    F: FnOnce(raw_window_handle::WindowHandle<'_>) -> T;

It runs f with the window's raw_window_handle::WindowHandle, returning None if there is no live OS window for the id (or the platform can't produce a handle). It wraps the existing internal with_window and mirrors the other WindowIdExt accessors (scale, is_visible, …), so it adds no new public types.

Why

Embedders sometimes need the parent window's native handle to attach a native child — for example embedding a wry child webview via WebViewBuilder::build_as_child(&handle). Today there is no public way to get it: the window registry (with_window) lives in the pub(crate) window::tracking module.

raw_window_handle::WindowHandle implements HasWindowHandle, so it plugs straight into wry/build_as_child and similar APIs. Exposing the raw-window-handle type (already a dependency) rather than the underlying winit/backend window keeps Floem from leaking its windowing-backend types — matching the design goal raised in #505 ("it would be a good thing if Floem doesn't directly expose winit's types"), and it should survive the planned winit → baseview abstraction since both back HasWindowHandle.

Example

use floem::window::WindowIdExt;

window_id.with_window_handle(|handle| {
    wry::WebViewBuilder::new()
        .with_url("https://example.com")
        .build_as_child(&handle)
});

Notes

• One method on the already-public WindowIdExt; no new public types, no winit leak.
• Compiles and cargo clippy --lib clean on current main.
• Used downstream to embed native browser tabs (a wry child webview) inside a Floem window.
• Related to the native-window interop discussed in Need a way to create a Floem NSView in a foreign Mac OS app #505 — this addresses the opposite direction (embedding a native child into a Floem window).

[jrmoulton]

I think this looks reasonable. Thanks!