Function collect_bool_word_scalar 

pub fn collect_bool_word_scalar<F>(len: usize, f: F) -> u64
where
    F: FnMut(usize) -> bool,

Packs up to 64 boolean values into a little-endian u64 word one bit at a time.

This is the scalar reference implementation behind collect_bool_word; prefer calling that entry point, which takes the SIMD fast path for full 64-bit words.