Function win_event_callback 

unsafe extern "system" fn win_event_callback(
    _hook: HWINEVENTHOOK,
    event: u32,
    hwnd: HWND,
    id_object: i32,
    id_child: i32,
    _id_event_thread: u32,
    _dwms_event_time: u32,
)

Phase B.9.1 — hook callback. Runs OUT-OF-CONTEXT (on the hooking thread, posted via the message pump). Must not block for long: we only do quick HWND-property reads and dispatch non-blocking sync IPC reports.