Function filter_cpu 

pub fn filter_cpu<T, F>(items: Vec<T>, pred: F) -> Vec<T>
where
    T: Send,
    F: Fn(&T) -> bool + Sync + Send,

Parallel filter with the same threshold rule as map_cpu (PAR-84).

Sequential when items.len() < CPU_MAP_THRESHOLD so small console/net buffers never pay Rayon coordination overhead.