add native window/webview builtins to el runtime

Adds five new builtins backed by wry 0.47 + tao 0.30:
- window_open(title, width, height) — configure window
- webview_load_url(url) — set URL to navigate to
- webview_load_html(html) — set HTML to display
- webview_eval(js) — queue JS to eval after load
- window_run() — open native OS window and run event loop (blocking, exits on close)

WebViewState is stored in a thread_local. window_run() calls
tao's event_loop.run() which never returns; process exits when
the window is closed via std::process::exit(0).

bin/el/Cargo.toml

@@ -34,3 +34,5 @@ hmac        = { workspace = true }
 sha2        = { workspace = true }
 hex         = { workspace = true }
 base64      = { workspace = true }
+wry         = "0.47"
+tao         = "0.30"