Function attach 

pub fn attach(vbox: &Box, mpv: *mut mpv_handle) -> bool

Start drawing mpv’s video underneath the webview.

vbox is Tauri’s default_vbox() — the container the webview already lives in. It is not modified; only a draw handler is added.

Must run on the GTK main thread.

TRACES: UR-080 | DR-231, DR-232, DR-233