Function apply_hwnd_visibility_changed 

pub fn apply_hwnd_visibility_changed(
    state: &mut State,
    hwnd: u64,
    visible: bool,
    now_ms: u64,
) -> Vec<Event>

Phase B.9.1 — handle Command::ReportHwndVisibilityChanged. Drift fires only on visible=false for a known label that has not been foregrounded since open AND is past the post-open placement grace window.