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/main/runtime/overlay-runtime-bootstrap.ts

@@ -31,6 +31,8 @@ type InitializeOverlayRuntimeCore = (options: {
   ) => Promise<KikuFieldGroupingChoice>;
   getKnownWordCacheStatePath: () => string;
   shouldStartAnkiIntegration: () => boolean;
+  bindOverlayOwner?: () => void;
+  releaseOverlayOwner?: () => void;
 }) => void;
 
 export function createInitializeOverlayRuntimeHandler(deps: {