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/overlay-window-config.test.ts

@@ -8,7 +8,32 @@ test('overlay window config explicitly disables renderer sandbox for preload com
     yomitanSession: null,
   });
 
+  assert.equal(options.backgroundColor, '#00000000');
   assert.equal(options.webPreferences?.sandbox, false);
+  assert.equal(options.webPreferences?.backgroundThrottling, false);
+});
+
+test('Windows visible overlay window config does not start as always-on-top', () => {
+  const originalPlatform = process.platform;
+
+  Object.defineProperty(process, 'platform', {
+    configurable: true,
+    value: 'win32',
+  });
+
+  try {
+    const options = buildOverlayWindowOptions('visible', {
+      isDev: false,
+      yomitanSession: null,
+    });
+
+    assert.equal(options.alwaysOnTop, false);
+  } finally {
+    Object.defineProperty(process, 'platform', {
+      configurable: true,
+      value: originalPlatform,
+    });
+  }
 });
 
 test('overlay window config uses the provided Yomitan session when available', () => {