Function apply_hwnd_foreground_changed 

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

Phase B.9.1 — handle Command::ReportHwndForegroundChanged. Updates the “has been seen” flag. Never emits drift directly — its role is to suppress future HiddenSinceOpen emissions.