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

package.json

@@ -113,6 +113,7 @@
     "commander": "^14.0.3",
     "hono": "^4.12.7",
     "jsonc-parser": "^3.3.1",
+    "koffi": "^2.15.6",
     "libsql": "^0.5.22",
     "ws": "^8.19.0"
   },