feat(windows): mpv draws Windows video into the app window

DR-237's video half. mpv now renders Windows video the way
tauri-plugin-libmpv does on Windows: MpvBackend is handed the main
window's HWND and sets it as `wid` before mpv initialises, so mpv draws
as a child of the app window beneath the transparent WebView2, with
vo=gpu-next,gpu. osc, default bindings, VO keyboard and cursor handling
are off, so the Svelte controls drawn over the picture are the only ones.

- video_output() decides per platform (UT-274): Window(hwnd) on Windows,
  RenderApi on Linux, Off without native video. Windows with no handle
  draws nothing rather than letting mpv open a top-level window.
- native_video::enabled() is true on Windows as well, so the frontend
  takes the native path there: NativePlayerAdapter, and the page clears
  its background while video is on screen.
- enableNativeVideoCompositing() no longer logs a missing Android bridge
  as an error on the desktop, where there is no bridge to have.

Verified: every option, wid included, is accepted by the real libmpv on
Linux and by the shipped Windows DLL under wine; the Windows unit suite
passes under wine (949). Not yet seen on real Windows hardware.

src-tauri/src/conformance_runner.rs

@@ -150,6 +150,7 @@ pub fn run_engine(url: &str, engine: Engine) -> u32 {
                     None,
                     std::sync::Arc::new(tokio::sync::Mutex::new(None)),
                     std::sync::Arc::new(crate::playback_reporting::throttle::EventThrottler::new()),
+                    None,
                 )
                 .expect("could not create the legacy backend"),
                 crate::player::media_player::Capabilities::mpv(),