Fix Windows overlay tracking, z-order, and startup visibility

- switch Windows overlay tracking to native win32 polling with native owner and z-order helpers
- keep the visible overlay and stats overlay aligned across focus handoff, transient tracker misses, and minimize/restore cycles
- start the visible overlay click-through and hide the initial opaque startup frame until the tracked transparent state settles
- add a backlog task for the inconsistent mpv y-t overlay toggle after menu toggles

src/core/services/index.ts

@@ -72,6 +72,7 @@ export {
   createOverlayWindow,
   enforceOverlayLayerOrder,
   ensureOverlayWindowLevel,
+  isOverlayWindowContentReady,
   syncOverlayWindowLayer,
   updateOverlayWindowBounds,
 } from './overlay-window';