Function filter_compact 

pub fn filter_compact<T, F>(data: &[T], predicate: F) -> Vec<T>
where
    T: Clone + Send + Sync,
    F: Fn(&T) -> bool + Sync,

Stream compaction: collect elements satisfying predicate into a new vec.

Mimics GPU stream compaction (prefix-sum + scatter). The output preserves the relative order of passing elements.